Communication management apparatus, communication node, and communication system, and data communication method

ABSTRACT

The communication management apparatus includes a network-presence-check processing unit that performs network presence check processing for generating network presence information, a token-circulation-order determining unit that determines token circulation order using the network presence information, a setup processing unit that notifies each of communication nodes in the network of token circulation destination information, a token-frame processing unit that, after transmission of a data frame by a data-frame-communication processing unit, transmits, based on the token circulation order, a token frame including information concerning a transmission right acquiring apparatus that acquires a transmission right next and determines whether the transmission right acquiring apparatus of the transmission right acquiring apparatus information of the received token frame is the own apparatus, and the data-frame-communication processing unit that performs transmission and reception processing for a data frame for acquiring the transmission right.

CROSS REFERENCE TO RELATED APPLICATION

This is a Continuation Application of U.S. application Ser. No.13/142,192 filed Jun. 24, 2011 which is a National Stage ofInternational Application No. PCT/JP2008/073631 filed Dec. 25, 2008, thecontents of all of which are incorporated herein by reference in theirentirety.

TECHNICAL FIELD

The present invention relates to a communication management apparatus, acommunication node, and a communication system, and a data communicationmethod for performing communication using a token frame amongcommunication nodes connected by an Ethernet.

BACKGROUND ART

In the past, an Ethernet, the same applies in the following explanation)is known as a system for performing transmission and reception of databetween communication nodes connected by a cable. As a connection formof communication nodes in the Ethernet, bus topology, star topology,line topology, and the like are known (see, for example, Non-PatentLiterature 1).

The bus topology is configured by, with one cable serving as a trunkline set as the center, extending branch cables from the center atappropriate intervals and arranging a plurality of communication nodes.In this bus topology, when the communication nodes simultaneouslytransmit data, collision of the data occurs. Therefore, a CSMA/CD(Carrier Sense Multiple Access/Collision Detection) system for, whileavoiding collision, securing a transmission right and propagating thedata to all connected communication nodes is adopted to performtransmission of the data.

However, as a connection form of communication nodes in recent years,the star topology is mainly used. This star topology has a configurationin which, with a communication node called switching hub having aplurality of ports set as the center, other communication nodes areconnected to the ports of the switching hub via a cable such as a UTPcable (Unshielded Twisted Pair Cable). The switching hub also has abuffer memory. Even when a plurality of communication nodessimultaneously transmit data, after accumulating the data in the buffermemory, the switching hub transmits the data to all ports other than aport at a transfer destination or an arrival port. Therefore, unlike thebus topology, it is unnecessary to take into account collision of thedata.

-   Non-Patent Literature: “Impress Standard Textbook Series Revised    Edition 10 Gigabit Ethernet Textbook”, compiled under the    supervision of Osamu Ishida and Koichiro Seto, Kabushiki Kaisha    Impress Network Business Company, Apr. 11, 2005

DISCLOSURE OF INVENTION Problem to be Solved by the Invention

In recent years, even in a network connecting FA (Factory Automation)apparatuses (hereinafter referred to as FA network) in which real-timeproperties are requested, it is a general practice to connect theapparatuses through the Ethernet and perform control. For example, acontrol system is established in which communication units ofprogrammable controllers that control control targets (hereinafterreferred to as master stations) are connected and a communication unitof a programmable controller (a master station) and communication unitsof input and output apparatuses (hereinafter referred to as slavestations) are connected by the Ethernet.

Because real-time properties are requested, the control system performs,within a predetermined time, a series of processing for transmittingdata from the slave stations to the master station and, in the masterstation, performing predetermined arithmetic processing using the datareceived from the slave stations to calculate control data andtransmitting the control data to the slave stations and repeatedlyperforms the series of processing.

In general, a large number of slave stations are connected to thecontrol system. A large number of data are transmitted from the slavestations to the master station in a short time. At this point, when thecontrol system adopts the star topology, even if the slave stationssimultaneously transmit data, the data is accumulated in a buffer memoryof a switching hub and transmitted to the master station in order.However, when a large number of slave stations are connected to thecontrol system, the number of data accumulated in the buffer memoryincreases and a delay occurs in the data relayed to the master station.In the worst case, there is a problem in that the data transmission tothe master station does not end within the predetermined time and thereal-time properties of the control system are spoiled. Discarding offrames due to exhaustion of the buffer memory of the switching huboccurs and the data from the slave stations is temporarily concentratedon the master station. Depending on processing speed of the masterstation, it is likely that a loss of frames occurs in the master stationas well.

The present invention has been devised in view of the above problems andit is an object of the present invention to obtain a communicationmanagement apparatus, a communication node, and a communication system,and a data communication method that can perform transmission andreception of frames without causing a data delay when the transmissionand reception of the frames is performed using the Ethernet.

Means for Solving Problem

In order to attain the object, a communication management apparatusaccording to the present invention is a communication managementapparatus that manages transmission of data in a network of the samesegment in which a plurality of communication nodes are connected in astar shape or a line shape by an Ethernet cable, the communicationmanagement apparatus including: a network-presence-check processing unitthat performs network presence check processing for acquiring, from thecommunication node, the communication nodes present in the network and aconnection relation among the communication nodes and generating networkpresence information; a token-circulation-order determining unit thatdetermines token circulation order using the network presenceinformation to minimize the number of times a token frame, which is atransmission right of data, passes a transmission line between anadjacent pair of the communication nodes; a setup processing unit thatnotifies, based on the token circulation order, each of thecommunication nodes in the network of information concerning a tokencirculation destination that is a communication node to which thetransmission right is given after the communication node; a token-frameprocessing unit that compares transmission right acquiring apparatusinformation in the received token frame and a MAC address of the ownapparatus to determine whether the transmission right is obtained and,when the transmission right is obtained, after transmission of a dataframe by a data-frame-communication processing unit, transmits a tokenframe in which, based on the token circulation order, the nextcommunication node is set in the transmission right acquiring apparatusinformation; and the data-frame-communication processing unit thattransmits the data frame when the transmission right is acquired andreceives data frames from the other communication nodes.

Effect of the Invention

According to the present invention, the token frame is circulated to thecommunication nodes in the order for minimizing the number of times thetoken frame passes the transmission line between the communicationnodes.

Therefore, there are effects that, when transmission and reception offrames is performed using the Ethernet in a network in which real-timeproperties are requested, it is possible to realize optimum real-timeperformance adjusted to a connection state of the network and performthe transmission and reception of the frames without causing a datadelay by preventing collision of the frames.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram of an example of a communication system inwhich communication is performed using a token according to a firstembodiment.

FIG. 2-1 is a schematic block diagram of a functional configuration of aring management station.

FIG. 2-2 is a schematic block diagram of a functional configuration of aslave station.

FIG. 3-1 is a diagram of an example of a format of a network presencecheck frame.

FIG. 3-2 is a diagram of an example of a format of a network presenceacknowledge frame.

FIG. 3-3 is a diagram of an example of a format of a setup frame.

FIG. 3-4 is a diagram of an example of a format of a setup responseframe.

FIG. 3-5 is a diagram of an example of a format of a token frame.

FIG. 4-1 is a schematic diagram of an example of an establishing methodfor a logical ring and a communication method employing a tokenaccording to the first embodiment (first).

FIG. 4-2 is a schematic diagram of the example of the establishingmethod for a logical ring and the communication method employing a tokenaccording to the first embodiment (second).

FIG. 4-3 is a schematic diagram of the example of the establishingmethod for a logical ring and the communication method employing a tokenaccording to the first embodiment (third).

FIG. 4-4 is a schematic diagram of the example of the establishingmethod for a logical ring and the communication method employing a tokenaccording to the first embodiment (fourth).

FIG. 4-5 is a schematic diagram of the example of the establishingmethod for a logical ring and the communication method employing a tokenaccording to the first embodiment (fifth).

FIG. 4-6 is a schematic diagram of the example of the establishingmethod for a logical ring and the communication method employing a tokenaccording to the first embodiment (sixth).

FIG. 4-7 is a schematic diagram of the example of the establishingmethod for a logical ring and the communication method employing a tokenaccording to the first embodiment (seventh).

FIG. 4-8 is a schematic diagram of the example of the establishingmethod for a logical ring and the communication method employing a tokenaccording to the first embodiment (eighth).

FIG. 4-9 is a schematic diagram of the example of the establishingmethod for a logical ring and the communication method employing a tokenaccording to the first embodiment (ninth).

FIG. 4-10 is a schematic diagram of the example of the establishingmethod for a logical ring and the communication method employing a tokenaccording to the first embodiment (tenth).

FIG. 4-11 is a schematic diagram of the example of the establishingmethod for a logical ring and the communication method employing a tokenaccording to the first embodiment (eleventh).

FIG. 4-12 is a schematic diagram of the example of the establishingmethod for a logical ring and the communication method employing a tokenaccording to the first embodiment (twelfth).

FIG. 4-13 is a schematic diagram of the example of the establishingmethod for a logical ring and the communication method employing a tokenaccording to the first embodiment (thirteenth).

FIG. 4-14 is a schematic diagram of the example of the establishingmethod for a logical ring and the communication method employing a tokenaccording to the first embodiment (fourteenth).

FIG. 4-15 is a schematic diagram of the example of the establishingmethod for a logical ring and the communication method employing a tokenaccording to the first embodiment (fifteenth).

FIG. 5-1 is a schematic diagram of an example of network presence checkframes transmitted from a ring management station X.

FIG. 5-2 is a schematic diagram of an example of network presenceacknowledge frames transmitted from slave stations A and B.

FIG. 5-3 is a schematic diagram of an example of a network presencecheck frame transmitted by the slave station B.

FIG. 5-4 is a schematic diagram of an example of network presenceacknowledge frames transmitted from slave stations C and D.

FIG. 5-5 is a schematic diagram of an example of a network presencecheck frame transmitted by the slave station D.

FIG. 5-6 is a schematic diagram of an example of a network presenceacknowledge frame transmitted from a slave station E.

FIG. 5-7 is a schematic diagram of an example of setup framestransmitted from the ring management station X to the slave stations.

FIG. 5-8 is a schematic diagram of an example of setup response framestransmitted from the slave stations.

FIG. 5-9 is a schematic diagram of an example of token frames generatedby communication nodes.

FIG. 6 is a diagram of an example of network presence informationgenerated by the ring management station X.

FIG. 7 is a flowchart for explaining an example of a processingprocedure for determining circulating order of a token.

FIG. 8-1 is a schematic diagram of an example of a creation process fora token circulation table (first).

FIG. 8-2 is a schematic diagram of the example of the creation processfor a token circulation table (second).

FIG. 8-3 is a schematic diagram of the example of the creation processfor a token circulation table (third).

FIG. 8-4 is a schematic diagram of the example of the creation processfor a token circulation table (fourth).

FIG. 8-5 is a schematic diagram of the example of the creation processfor a token circulation table (fifth).

FIG. 8-6 is a schematic diagram of the example of the creation processfor a token circulation table (sixth).

FIG. 8-7 is a schematic diagram of the example of the creation processfor a token circulation table (seventh).

FIG. 8-8 is a schematic diagram of the example of the creation processfor a token circulation table (eighth).

FIG. 8-9 is a schematic diagram of the example of the creation processfor a token circulation table (ninth).

FIG. 9-1 is a diagram of an example of a processing procedure of amethod of establishing a logical ring when a slave station is added tothe communication system anew (first).

FIG. 9-2 is a diagram of the example of the processing procedure of themethod of establishing a logical ring when a slave station is added tothe communication system anew (second).

FIG. 9-3 is a diagram of the example of the processing procedure of themethod of establishing a logical ring when a slave station is added tothe communication system anew (third).

FIG. 9-4 is a diagram of the example of the processing procedure of themethod of establishing a logical ring when a slave station is added tothe communication system anew (fourth).

FIG. 9-5 is a diagram of the example of the processing procedure of themethod of establishing a logical ring when a slave station is added tothe communication system anew (fifth).

FIG. 9-6 is a diagram of the example of the processing procedure of themethod of establishing a logical ring when a slave station is added tothe communication system anew (sixth).

FIG. 9-7 is a diagram of the example of the processing procedure of themethod of establishing a logical ring when a slave station is added tothe communication system anew (seventh).

FIG. 9-8 is a diagram of the example of the processing procedure of themethod of establishing a logical ring when a slave station is added tothe communication system anew (eighth).

FIG. 9-9 is a diagram of the example of the processing procedure of themethod of establishing a logical ring when a slave station is added tothe communication system anew (ninth).

FIG. 9-10 is a diagram of the example of the processing procedure of themethod of establishing a logical ring when a slave station is added tothe communication system anew (tenth).

FIG. 9-11 is a diagram of the example of the processing procedure of themethod of establishing a logical ring when a slave station is added tothe communication system anew (eleventh).

FIG. 10 is a schematic diagram of an example of a network presenceacknowledge frame transmitted from a slave station F.

FIG. 11 is a diagram of an example of network presence information afterthe network presence acknowledge frame is received from the slavestation F.

FIG. 12 is a schematic diagram of an example of setup frames transmittedfrom the ring management station X to the slave stations C and F.

FIG. 13 is a diagram of an example of the structure of network presenceinformation according to a third embodiment.

FIG. 14 is a flowchart for explaining an example of a reconfigurationprocessing for a logical ring by a ring management station.

FIG. 15 is a schematic block diagram of an example of a functionalconfiguration of a ring management station according to a fourthembodiment.

FIG. 16-1 is a schematic diagram of an example of a procedure of reissueprocessing for a token frame according to the fourth embodiment (first).

FIG. 16-2 is a schematic diagram of the example of the procedure of thereissue processing for a token frame according to the fourth embodiment(second).

FIG. 16-3 is a schematic diagram of the example of the procedure of thereissue processing for a token frame according to the fourth embodiment(third).

FIG. 16-4 is a schematic diagram of the example of the procedure of thereissue processing for a token frame according to the fourth embodiment(fourth).

FIG. 16-5 is a schematic diagram of the example of the procedure of thereissue processing for a token frame according to the fourth embodiment(fifth).

FIG. 16-6 is a schematic diagram of the example of the procedure of thereissue processing for a token frame according to the fourth embodiment(sixth).

FIG. 16-7 is a schematic diagram of the example of the procedure of thereissue processing for a token frame according to the fourth embodiment(seventh).

FIG. 16-8 is a schematic diagram of the example of the procedure of thereissue processing for a token frame according to the fourth embodiment(eighth).

FIG. 17-1 is a schematic diagram of another example of the procedure ofthe reissue processing for a token frame according to the fourthembodiment (first).

FIG. 17-2 is a schematic diagram of the other example of the procedureof the reissue processing for a token frame according to the fourthembodiment (second).

FIG. 17-3 is a schematic diagram of the other example of the procedureof the reissue processing for a token frame according to the fourthembodiment (third).

FIG. 17-4 is a schematic diagram of the other example of the procedureof the reissue processing for a token frame according to the fourthembodiment (fourth).

FIG. 17-5 is a schematic diagram of the other example of the procedureof the reissue processing for a token frame according to the fourthembodiment (fifth).

FIG. 18 is a schematic block diagram of an example of a functionalconfiguration of a slave station according to a fifth embodiment.

FIG. 19-1 is a schematic diagram of an example of a procedure ofdetermination processing for leaving off the slave station andreconfiguration processing for a logical ring according to the fifthembodiment (first).

FIG. 19-2 is a schematic diagram of the example of the procedure of thedetermination processing for leaving off the slave station andreconfiguration processing for a logical ring according to the fifthembodiment (second).

FIG. 19-3 is a schematic diagram of the example of the procedure of thedetermination processing for leaving off the slave station andreconfiguration processing for a logical ring according to the fifthembodiment (third).

FIG. 19-4 is a schematic diagram of the example of the procedure of thedetermination processing for leaving off the slave station andreconfiguration processing for a logical ring according to the fifthembodiment (fourth).

FIG. 19-5 is a schematic diagram of the example of the procedure of thedetermination processing for leaving off the slave station andreconfiguration processing for a logical ring according to the fifthembodiment (fifth).

FIG. 19-6 is a schematic diagram of the example of the procedure of thedetermination processing for leaving off the slave station andreconfiguration processing for a logical ring according to the fifthembodiment (sixth).

FIG. 19-7 is a schematic diagram of the example of the procedure of thedetermination processing for leaving off the slave station andreconfiguration processing for a logical ring according to the fifthembodiment (seventh).

FIG. 19-8 is a schematic diagram of the example of the procedure of thedetermination processing for leaving off the slave station andreconfiguration processing for a logical ring according to the fifthembodiment (eighth).

FIG. 19-9 is a schematic diagram of the example of the procedure of thedetermination processing for leaving off the slave station andreconfiguration processing for a logical ring according to the fifthembodiment (ninth).

FIG. 19-10 is a schematic diagram of the example of the procedure of thedetermination processing for leaving off the slave station andreconfiguration processing for a logical ring according to the fifthembodiment (tenth).

EXPLANATIONS OF LETTERS OR NUMERALS

-   -   A to F Slave stations    -   X Ring management station    -   11-1, 11-2, 51-1, 51-2 Ports    -   20, 20A, 60, 60A Communication processing units    -   21, 65 Timers    -   22 Network-presence-check processing unit    -   23 Network-presence-information storing unit    -   24 Token-circulation-order determining unit    -   25 Token-circulation-order-information storing unit    -   26 Setup processing unit    -   27, 63 Token-frame processing units    -   28, 64 Data-frame-communication processing units    -   29 Network monitoring unit    -   30 Frame storing unit    -   51 Port    -   61 Control-frame response unit    -   62 Token-circulation-destination-information storing unit    -   66 Reset processing unit    -   101 Switching hub    -   102 Cable    -   111, 112 Transmission lines    -   200 Network presence check frame    -   220 Network presence acknowledge frame    -   240 Setup frame    -   260 Setup response frame    -   280 Token frame

BEST MODE(S) FOR CARRYING OUT THE INVENTION

Preferred embodiments of a communication management apparatus, acommunication node, and a communication system, and a data communicationmethod according to the present invention are explained in detail belowwith reference to the accompanying drawings. The present invention isnot limited by these embodiments.

First Embodiment

FIG. 1 is a schematic diagram of an example of a communication system inwhich communication is performed using a token according to a firstembodiment. This communication system includes a network of the samesegment in which a plurality of communication nodes are connected in aline shape or a star shape by an Ethernet, the same applies in thefollowing explanation). Each of the communication nodes includes twoports. The ports of the communication nodes are connected via a cable102 capable of performing full duplex communication such as a twist paircable or an optical fiber. In this example, as the communication node,one ring management station X functioning as a communication managementapparatus that manages transmission and reception of data (frames) inthe network of the same segment and five slave stations A to E thatperform, based on setting by the ring management station X, transmissionof data (frames) are provided.

As shown in FIG. 1, the ring management station X and the slave stationsA and B are connected in a line shape. Specifically, a first port X1 ofthe ring management station X and a second port A2 of the slave stationA are connected and a second port X2 of the ring management station Xand a first port B1 of the slave station B are connected.

The slave stations B, C, and D are connected in a star shape by aswitching hub 101. Specifically, a second port B2 of the slave stationB, a first port C1 of the slave station C, and a first port D1 of theslave station D are connected via the switching hub 101.

The slave stations D and E are connected in a line shape. Specifically,a second port D2 of the slave station D and a first port E1 of the slavestation E are connected.

MAC (Media Access Control) addresses (in the figures, represented asMAC_AD) of the communication nodes are set as follows:

ring management station X=100

slave station A=1

slave station B=2

slave station C=3

slave station D=4

slave station E=5

In the first embodiment, in such a communication system in which thecommunication nodes are connected by the Ethernet in a star shape or aline shape, the communication nodes do not freely perform transmissionof data, a frame for obtaining a data transmission right called token (atoken frame) is transmitted to the nodes in the communication system inorder, and the communication node that acquires the token can performtransmission of data to the other communication nodes. Transmissionorder of the token frame is, for example, as indicated by (A) below.

Ring management station X→slave station B→slave station C→slave stationD→slave station E→slave station A→ring management station X (A)

In this way, the communication system does not have a ring configurationin a physical network configuration. However, the communication systemis configured to circulate the data transmission right (the token frame)in order among the communication nodes in the communication system andreturn the transmission right to the ring management station X tothereby repeat the transmission right in a logical ring configuration.

FIG. 2-1 is a schematic block diagram of a functional configuration of aring management station. The ring management station X includes twoports 11-1 and 11-2 for connecting an Ethernet cable between the ringmanagement station and the adjacent communication nodes (the slavestations A to E) or the switching hub 101 and a communication processingunit 20 that performs, for example, transmission and receptionprocessing for frames via the ports 11-1 and 11-2 and processing forestablishing transmission order of the token frame.

The ports 11-1 and 11-2 include two ports of a first port 11-1 and asecond port 11-2. At least one port of these two ports 11-1 and 11-2only has to be connected to a port of a slave station adjacent thereto(or a port of a slave station via a switching hub).

The communication processing unit 20 includes a timer 21, anetwork-presence-check processing unit 22, anetwork-presence-information storing unit 23, a token-circulation-orderdetermining unit 24, a token-circulation-order-information storing unit25, a setup processing unit 26, a token-frame processing unit 27, and adata-frame-communication processing unit 28.

The timer 21 is started by a processing unit in the communicationprocessing unit 20 and has a function of measuring a predetermined time.In the first embodiment, the timer 21 counts time until thepredetermined time elapses after a network presence check frame istransmitted by the network-presence-check processing unit 22.

The network-presence-check processing unit 22 performs, after a powersupply for the own apparatus (the ring management station X) is turnedon or after a predetermined state occurs, network presence checkprocessing for detecting a connection state of the communication nodesincluded in the communication system (the network of the same segment).Specifically, the network-presence-check processing unit 22 creates anetwork presence check frame and transmits the network presence checkframe in broadcast and generates network presence information frominformation included in a network presence acknowledge frame, which is aresponse to a network presence check frame from a communication nodepresent in the communication system. This processing is performed untilthe predetermined time elapses after the timer 21 is started when thenetwork presence check frame is transmitted.

FIG. 3-1 is a diagram of an example of a format of the network presencecheck frame. A network presence check frame 200 is an Ethernet frame andincludes a destination MAC address (hereinafter referred to as DA) 201,a transmission source MAC address (hereinafter referred to as SA) 202,an Ethernet type (type) 203, data 204 in which data of upper layers isstored, and an FCS (Frame Check Sequence) 208 in which a check resultconcerning whether an error is present in information stored in the DA201 to the data 204 of the own frame is stored.

In the first embodiment, frame type information 205, MAC addressinformation 206 of the ring management station, and transmission portinformation 207 of the network presence check frame of the own stationare stored in a part of the data 204.

The frame type information 205 is information for identifying what kindof a frame the own Ethernet frame is. Information indicating that aframe is the network presence check frame is stored in the frame typeinformation 205. In this example, the network presence check frame isrepresented as “TestData”.

A MAC address of the ring management station X is stored in the MACaddress information 206 of the ring management station. Port informationindicating from which port a communication node transmits the networkpresence check frame 200 is stored in the transmission port information207 of the network presence check frame of the own station.

FIG. 3-2 is a diagram of an example of a format of the network presenceacknowledge frame. This network presence acknowledge frame 220 is alsoan Ethernet frame. Information used in the first embodiment is definedin data 224. Specifically, frame type information 225, SA information226 in the received network presence check frame, and port information227 of a station that transmits the network presence check frame arestored in a part of the data 224.

Information indicating that a frame is the network presence acknowledgeframe 220 is stored in the frame type information 225. In thisspecification, the network presence acknowledge frame is represented as“TestDataACK”. A MAC address stored in the SA 202 area of the networkpresence check frame 200 received by a communication node (a slavestation) is stored in the “SA information in the received networkpresence check frame” 226. Further, port information stored in the“transmission port information of the network presence check frame ofthe own station” 207 of the data 204 area in the network presence checkframe 200 received by the communication node (the slave station) isstored in the “port information of the station that transmits thenetwork presence check frame” 227.

When the network-presence-check processing unit 22 receives the networkpresence acknowledge frame 220, the network-presence-check processingunit 22 generates network presence information in which the “SAinformation in the received network presence check frame” 226 and the“port information of the station that transmits the network presencecheck frame” 227 are associated with an “SA” 222 of the received networkpresence acknowledge frame 220 and stores the network presenceinformation in the network-presence-information storing unit 23.

The network-presence-information storing unit 23 stores the networkpresence information generated by the network-presence-check processingunit 22. As explained above, this network presence information includesa MAC address (the SA 222) of a communication node (a slave station)that transmits the network presence acknowledge frame 220, the “SAinformation in the received network presence check frame” 226, and the“port information of the station that transmits the network presencecheck frame” 227.

After the timer 21 started by the network-presence-check processing unit22 times out, the token-circulation-order determining unit 24 grasps aconnection state among the communication nodes included in thecommunication system using the network presence information stored inthe network-presence-information storing unit 23 and performs processingfor configuring a logical ring, i.e., processing for determiningcirculation order of a token frame. In the processing for configuring alogical ring, the token-circulation-order determining unit 24establishes a logical ring such that the number of communication nodesthat a token as a transmission right passes while circulating once isminimized (when a cable (including a switching hub) connecting twocommunication nodes is referred to as a transmission line, the number oftransmission lines that the token passes while circulating once isminimized). As a method of determining circulation order of the tokenthat satisfies such a condition, the communication nodes only have to bedrawn with a single stroke of the brush in the network belonging to thesame segment. A method of determining the circulation order of the tokenby this method of the drawing with a single stroke of the brush isexplained later. The determined circulation order of the token frame isstored in the token-circulation-order-information storing unit 25 astoken circulation order information.

When the token circulation order information is determined by thetoken-circulation-order determining unit 24, the setup processing unit26 generates, with respect to each of the communication nodes (slavestations) in the communication system, a setup frame includinginformation concerning a communication node to which the transmissionright is given after the communication using the token circulation orderinformation and transmits the setup frame to the communication nodes.The setup processing unit 26 determines whether setup response frames,which are responses to the setup frame, are received from all thecommunication nodes. When the setup response frames are received fromall the communication nodes, the setup processing unit 26 notifies thetoken-frame processing unit 27 to that effect.

FIG. 3-3 is a diagram of an example of a format of the setup frame. Thissetup frame 240 is also an Ethernet frame. Frame type information 245and token frame destination MAC address setting information 246 arestored in a part of a data 244 area.

Because this setup frame 240 is set with respect to the communicationnodes, a MAC address of a slave station that the ring management stationX is about to set is set in a DA 241. Information indicating that aframe is the setup frame 240 is stored in the frame type information245. In this specification, the setup frame 240 is represented as“Setup”. When the transmission right is controlled by a token frame, aMAC address of a slave station that is given the transmission rightafter a communication node at a transmission destination of this setupframe 240 or the ring management station X is stored in the token framedestination MAC address setting information 246.

FIG. 3-4 is a diagram of an example of a format of the setup responseframe. This setup response frame 260 is also an Ethernet frame. Frametype information 265 indicating that the own frame is the setup responseframe 260 is stored in a part of a data 264 area. The setup responseframe 260 is represented as “SetupACK”.

The setup processing unit 26 can check, for example, by setting a flagindicating that the setup response frame 260 is received in the networkpresence information of the network-presence-information storing unit23, whether the setup response frame 260 is received from all thecommunication nodes in the communication system.

When it is notified by the setup processing unit 26 that the setupresponse frame 260 is received from all the communication nodes in thecommunication system, the token-frame processing unit 27 generates atoken frame according to the token circulation order information of thetoken-circulation-order-information storing unit 25 and transmits thetoken frame from all the ports 11-1 and 11-2 of the own station inbroadcast.

FIG. 3-5 is a diagram of an example of a format of the token frame. Thistoken frame 280 is also an Ethernet frame. Frame type information 285and MAC address information for token-addressed-to-own-stationdetermination 286 are stored in a part of a data 284 area.

Information indicating that a frame is the token frame 280 is stored inthe frame type information 285. In this specification, the token frame280 is represented as “Token”. A MAC address of a communication nodehaving the transmission right is stored in the MAC address informationfor token-addressed-to-own-station determination 286.

When the token-frame processing unit 27 receives the token frame 280transmitted from another communication node, the token-frame processingunit 27 compares the MAC address information fortoken-addressed-to-own-station determination 286 in the data 284 of thetoken frame 280 and a MAC address of the own station (the ringmanagement station X). When the MAC address information fortoken-addressed-to-own-station determination 286 and the MAC addresscoincide with each other, the token-frame processing unit 27 determinesthat the transmission right is obtained. The transmission processing fora data frame by the data-frame-communication processing unit 28 isperformed. When the MAC address information fortoken-addressed-to-own-station determination 286 and the MAC address donot coincide with each other, the token-frame processing unit 27determines that the transmission right is not obtained yet. In both thecases, the received token frame 280 is transferred (repeated) fromanother port that is not the port where the token frame 280 is received.

The data-frame-communication processing unit 28 performs transmissionand reception processing for a data frame. For example, in an FAnetwork, the data-frame-communication processing unit 28 calculates, ata predetermined period, data set in the slave stations A to E, convertsthe data into a data frame, and transmits the data frame to the slavestations A to E. The data-frame-communication processing unit 28 alsohas a function of receiving data frames transmitted from the slavestations A to E and transferring (repeating) data frames addressed toother slave stations by the slave stations A to E.

“TestData”, “TestDataACK”, and the like are stored in the frame typeinformation 205, 225, 245, 265, and 285 of the frames shown in FIGS. 3-1to 3-5 to identify the respective frames. However, numerical values foruniquely identifying the frames can be set for the respective frames andstored in the frame type information 205, 225, 245, 265, and 285.

FIG. 2-2 is a schematic block diagram of a functional configuration of aslave station. The slave stations A to E include two ports 51-1 and 51-2for connecting an Ethernet cable between each of the slave stations andthe adjacent communication nodes (the ring management station X and theslave stations) or the switching hub 101 and a communication processingunit 60 that performs transmission and reception processing for framesvia the ports 51-1 and 51-2.

As in the ring management station X, the ports 51-1 and 51-2 include twoports of a first port 51-1 and a second port 51-2. At least one port ofthese two ports 51-1 and 51-2 only has to be connected to acommunication node.

The communication processing unit 60 includes a control-frame responseunit 61, a token-circulation-destination-information storing unit 62, atoken-frame processing unit 63, and a data-frame-communicationprocessing unit 64.

The control-frame response unit 61 performs response from the ringmanagement station X to control frames such as the network presencecheck frame 200 and the setup frame 240. For example, when thecontrol-frame response unit 61 receives the network presence check frame200, the control-frame response unit 61 generates the network presenceacknowledge frame 220 shown in FIG. 3-2 and returns the network presenceacknowledge frame 220 to the ring management station X. When thecontrol-frame response unit 61 receives the setup frame 240, thecontrol-frame response unit 61 extracts the token frame destination MACaddress setting information 246 in the data 244 of the setup frame 240and stores the token frame destination MAC address setting information246 in the token-circulation-destination-information storing unit 62 astoken circulation destination information. The control-frame responseunit 61 generates the setup response frame 260 shown in FIG. 3-4 andreturns the setup response frame 260 to the ring management station X.In this specification, a frame exchanged between the ring managementstation X and the slave stations A to E in the network presence checkprocessing and the logical ring configuration processing is referred toas control frame. A frame transmitted by acquiring a token frame after alogical ring is configured is referred to as data frame.

The control-frame response unit 61 also has a function of, according toa frame type of a control frame received from the ring managementstation X or another slave station, reconfiguring and transmitting aframe or simply repeating a frame. For example, when the networkpresence check frame 200 is received from the ring management station X,if a transmission line is established in a port other than a port wherethe frame is received, the control-frame response unit 61 performsprocessing for rewriting the SA 202 shown in FIG. 3-1 of the receivednetwork presence check frame and transmission port information 207 ofthe network presence check frame of the own station in the data 204,reconfigures a network presence check frame, and outputs the networkpresence check frame from a port other than the reception port.

The control-frame response unit 61 has a function of, for example, whena control frame including the setup frame 240 from the ring managementstation X or the network presence acknowledge frame 220 and the setupresponse frame 260 from another slave station is received, performing noprocessing for the frame and simply repeating the frame.

The token-circulation-destination-information storing unit 62 stores aMAC address of a communication node that obtains the transmission rightafter the own communication node (slave station). As the MAC address, asexplained above, contents of the token frame destination MAC addresssetting information 246 in the data 244 of the setup frame 240 arestored. In the first embodiment, only a MAC address of a communicationnode to which a token should be transmitted next is stored. Therefore,it is possible to hold down a data amount compared with the tokencirculation order storage information stored by the ring managementstation X.

When the token-frame processing unit 63 receives the token frame 280transmitted from another communication node, the token-frame processingunit 63 compares the MAC address information fortoken-addressed-to-own-station determination 286 in the data 284 of thetoken frame 280 and a MAC address of the own station (slave station).When the MAC address information for token-addressed-to-own-stationdetermination 286 and the MAC address coincide with each other, thetoken-frame processing unit 63 determines that the transmission right isobtained. Transmission processing for a data frame by thedata-frame-communication processing unit 64 is performed. Thereafter,the token-frame processing unit 63 generates, in the MAC addressinformation for token-addressed-to-own-station determination 286 in thedata 284 of the token frame 280, the token frame 280 in which the tokencirculation destination information stored in thetoken-circulation-destination-information storing unit 62 is stored andtransmits the token frame 280 from all the ports 51-1 and 51-2 of theown station in broadcast. On the other hand, when the MAC addressinformation for token-addressed-to-own-station determination 286 in thedata 284 of the received token frame 280 and the MAC address of the ownstation (slave station) do not coincide with each other, the token-frameprocessing unit 63 determines that the transmission right is notobtained yet. In both the cases, the received token frame 280 isrepeated to another port that is not a port where the token frame 280 isreceived.

The data-frame-communication processing unit 64 performs transmissionand reception processing for a data frame. Specifically, thedata-frame-communication processing unit 64 performs transmission andreception processing for a data frame between the ring managementstation X and the other slave stations.

An establishing method for a logical ring in such a communication systemand a communication method employing a token are explained below. FIGS.4-1 to 4-15 are schematic diagrams of an example of an establishingmethod for a logical ring and a communication method employing a tokenaccording to the first embodiment.

First, as shown in FIG. 4-1, after the slave stations A to E areconnected to the ring management station X by an Ethernet cable, powersupplies for the slave stations A to E are turned on. In this state, theslave stations A to E wait for reception of a network presence checkframe from the ring management station X.

Thereafter, when a power supply for the ring management station X isturned on, the ring management station X recognizes slave stationsconnected on the network of the same segment including the ringmanagement station X and, to establish a logical ring, performsprocessing explained below. First, after starting the timer 21, as shownin FIG. 4-2, the network-presence-check processing unit 22 of the ringmanagement station X generates a network presence check frame (in thefigure, represented as TestDataFrame) and transmits the network presencecheck frame from all the ports X1 and X2 in broadcast.

FIG. 5-1 is a schematic diagram of an example of a network presencecheck frame transmitted from the ring management station X. In a networkpresence check frame transmitted from the first port X1 of the ringmanagement station X, as indicated by “TestDataFrame(X1→all)”, abroadcast address (for example, 2-byte notation, “FFFF (all F)” is setin a DA, a MAC address “100” of the ring management station X is set inan SA, “TestData” is stored in frame type information, the MAC address“100” of the own station is stored in MAC address information of a ringmanagement station, and “port_X1” indicating the first port X1 is set intransmission port information of network presence check frame of the ownstation.

In a network presence check frame transmitted from the second port X2 ofthe ring management station X, as indicated by “TestDataFrame(X2→all)”,items other than the transmission port information of a network presencecheck frame of the own station are the same as those of the networkpresence check frame “TestDataFrame(X1→all)” transmitted from the firstport X1. “port_X2” indicating the second port X2 is set in transmissionport information of a network presence check frame of the own station.

In this way, the network presence check frame transmitted from the firstport X1 of the ring management station X is received in the second portA2 of the slave station A and the network presence check frametransmitted from the second port X2 of the ring management station X isreceived in the first port B1 of the slave station B.

As shown in FIG. 4-3, the control-frame response units 61 of the slavestations A and B, which receive the network presence check frame fromthe ring management station X, generate network presence acknowledgeframes (in the figure, represented as TestDataACKFrame) and return thenetwork presence acknowledge frame to the ring management station X.

FIG. 5-2 is a schematic diagram of an example of network presenceacknowledge frames transmitted from the slave stations A and B. In thenetwork presence acknowledge frame transmitted from the second port A2of the slave station A, as indicated by “TestDataACKFrame(A2→X)”, theMAC address “100” of the ring management station X is set in a DA, a MACaddress “1” of the own station is set in an SA, and “TestDataACK” isstored in frame type information. In SA information of the receivednetwork presence check frame and port information of a station thattransmits the network presence check frame, “100” and “port_X1” arerespectively set with reference to the SA and the transmission portinformation of the network presence check frame of the own station ofthe received network presence check frame “TestDataFrame(X1→all)” shownin FIG. 5-1.

In a network presence acknowledge frame transmitted from the first portB1 of the slave station B, as indicated by “TestDataACKFrame(B1→X)”, aMAC address “2” of the own station is set in a SA. “port_X2” is set inport information of a station that transmits the network presence checkframe with reference to transmission port information of the networkpresence check frame of the own station of the network presence checkframe “TestDataFrame(X2→all)” shown in FIG. 5-1. Otherwise, the networkpresence acknowledge frame is the same as “TestDataACKFrame(A2→X)”.

When the slave station A that receives the network presence check framefrom the ring management station X returns a network presenceacknowledge frame to the ring management station X, the slave station Achanges to a state of waiting for setting from the ring managementstation X.

As shown in FIG. 4-4, the control-frame response unit 61 of the slavestation B that receives the network presence check frame from the ringmanagement station X generates a network presence check frame (in thefigure, represented as TestDataFrame(B2→all)) obtained by rewriting thenetwork presence check frame received from the first port B1 andattempts to transmit the network presence check frame to the second portB2 of the own station other than the first port B1. As a result, when atransmission line is established in the second port B2, the networkpresence check frame is transmitted. When a transmission line is notestablished, the network presence check frame is not transmitted.Because a transmission line is established in the second port B2, thecontrol-frame response unit 61 performs processing for transmitting thenetwork presence check frame from the second port B2.

FIG. 5-3 is a schematic diagram of an example of the network presencecheck frame transmitted by the slave station B. As shown in the figure,the network presence check frame “TestDataFrame(B2→all)” is a frameobtained by rewriting the SA of the received network presence checkframe “TestDataFrame(X2→all)” to the MAC address “2” of the own stationand rewriting transmission port information of the network presencecheck frame of the own station to “port_B2”.

When the slave station C receives a network presence check frame fromthe first port C1, as shown in FIG. 4-5, the slave station C returns anetwork presence acknowledge frame, which is a response to the networkpresence check frame, from the first port C1. Like the slave station C,when the slave station D receives a network presence check frame fromthe first port D1, the slave station D returns a network presenceacknowledge frame, which is a response to the network presence checkframe, from the port D1.

FIG. 5-4 is a schematic diagram of an example of network presenceacknowledge frames transmitted from the slave stations C and D. In anetwork presence acknowledge frame transmitted from the first port C1 ofthe slave station C, as indicated by “TestDataACKFrame(C1→X)”, a MACaddress “3” of the own station is set in an SA, “TestDataACK” is storedin frame type information, and “100”, “2”, and “port_B2” arerespectively set in a DA, SA information in the received networkpresence check frame, and port information of a station, which transmitsthe network frame presence check frame, with reference to the MACaddress of the ring management station, the SA, and the transmissionport information of the network presence check frame of the own stationof the received network presence check frame “TestDataFrame(B2→all)”shown in FIG. 5-3.

A network presence acknowledge frame is transmitted from the first portD1 of the slave station D as well. As indicated by“TestDataACKFrame(D1→X)”, this network presence acknowledge frame hasthe same contents as the network presence acknowledge frame“TestDataACKFrame(C1→X)” transmitted from the slave station C exceptthat a MAC address “4” of the own station is set in an SA.

Thereafter, the control-frame response unit 61 of the slave station Bdetermines frame type information of the network presence acknowledgeframes received from the slave stations C and D. Because the frame typeinformation is the network presence acknowledge frame, the control-frameresponse unit 61 directly repeats the network presence acknowledgeframes to the ring management station X.

When the slave station C that receives the network presence check framefrom the slave station B returns the network presence acknowledge frameto the ring management station X, the slave station C changes to a stateof waiting for setting from the ring management station X.

As shown in FIG. 4-6, the control-frame response unit 61 of the slavestation D that receives the network presence check frame from the slavestation B generates a network presence check frame (in the figure,represented as TestDataFrame(D2→all)) obtained by rewriting the networkpresence check frame received from the first port D1 and attemptstransmission of the network presence check frame to the second port D2of the own station other than the first port D1. As a result, when atransmission line is established in the second port D2, the networkpresence check frame is transmitted. When a transmission line is notestablished, the network presence check frame is not transmitted.Because a transmission line is established in the second port D2, thecontrol-frame response unit 61 performs processing for outputting thenetwork presence check frame from the second port D2.

FIG. 5-5 is a schematic diagram of an example of the network presencecheck frame transmitted by the slave station D. As shown in the figure,the network presence check frame “TestDataFrame(D2→all)” is a networkpresence check frame obtained by rewriting the SA of the receivednetwork presence check frame “TestDataFrame(B2→all)” to the MAC address“4” of the own station and rewriting the transmission port informationof the network presence check frame of the own station to “port_D2”.

When the slave station E receives a network presence check frame fromthe first port E1, as shown in FIG. 4-7, the slave station E returns anetwork presence acknowledge frame, which is a response to the networkpresence check frame, (in the figure, represented asTestDataACKFrame(E1→X)) from the first port E1.

FIG. 5-6 is a schematic diagram of an example of the network presenceacknowledge frame transmitted from the slave station E. In the networkpresence acknowledge frame transmitted from the first port E1 of theslave station E, a MAC address “5” of the own station is set in an SA,“TestDataACK” is stored in frame type information, and “100”, “4”, and“port_D2” are respectively set in a DA, SA information in the receivednetwork presence check frame, and port information of a station, whichtransmits the network presence check frame, with reference to the MACaddress of the ring management station, the SA, and the transmissionport information of the network presence check frame of the own stationof the received network presence check frame “TestDataFrame(D2→all)”shown in FIG. 5-5.

The control-frame response units 61 of the slave stations D and Bdetermine frame type information of the network presence acknowledgeframe transmitted from the slave station E. Because the frame typeinformation is the network presence acknowledge frame, the control-frameresponse unit 61 performs processing for directly repeating the networkpresence acknowledge frames to the ring management station X.

When the slave station E that receives the network presence check framefrom the slave station D returns the network presence acknowledge frameto the ring management station X, the slave station E changes to a stateof waiting for setting from the ring management station X.

It is assumed that the network-presence-check processing unit 22 of thering management station X receives the network presence acknowledgeframes from the slave stations A to E, which are the communication nodesin the network of the same segment, during the start of the timer 21when a network presence acknowledge frame set in FIG. 4-1 is waited.

When the network-presence-check processing unit 22 of the ringmanagement station X receives the network presence acknowledge framesfrom the slave stations A to E during the start of the timer 21, thenetwork-presence-check processing unit 22 generates network presenceinformation from the frame and stores the network presence informationin the network-presence-information storing unit 23.

FIG. 6 is a diagram of an example of network presence informationgenerated by the ring management station X. This network presenceinformation includes items of an SA, SA information in a receivednetwork presence check frame, and port information of a station thattransmits the network presence check frame. The network-presence-checkprocessing unit 22 of the link management station X acquires respectivekinds of information from areas in which the items are defined in thereceived network presence acknowledge frame and stores the informationin the network presence information.

Thereafter, when the timer 21 detects that the predetermined time forwaiting for a network presence acknowledge frame elapses, thetoken-circulation-order determining unit 24 of the ring managementstation X performs processing for configuring a logical ring usingnetwork presence information accumulated in thenetwork-presence-information storing unit 23 until the timer 21 timesout. The ring management station X establishes a logical ring such thatthe number of communication nodes that a token as a transmission rightpasses while circulating once is, for example, minimized (when the cable102 (including a cable in which the switching hub 101 is includedbetween cables 102) connecting two communication nodes is referred to astransmission line, such that the number of transmission lines that atoken frame passes while circulating once is, for example, minimized).As a method of determining circulation order of a token frame thatsatisfies such a condition, communication nodes only have to beconnected by drawing with a single stroke of the brush (in such a manneras to turn around a tree) in a network belonging to the same segment. Amethod of determining circulation order of a token with this method ofthe drawing with a single stroke of the brush is explained below.

FIG. 7 is a flowchart for explaining an example of a processingprocedure for determining circulation order of a token. First, thetoken-circulation-order determining unit 24 of the ring managementstation X selects a MAC address of the own station (step S11) andselects one port among ports of the own station (step S12). Thetoken-circulation-order determining unit 24 registers a combination ofthe selected MAC address of the own station and the selected port in atoken circulation table, which is token circulation order information,of the token-circulation-order-information storing unit 25 (step S13).

Subsequently, the token-circulation-order determining unit 24 performs asearch in order to determine whether a record of a combination of “SAinformation in the received network presence check frame” and “portinformation of a station that transmits the network presence checkframe”, which is the same as the combination of the selected MAC addressand the selected port, is present in the network presence information ofthe network-presence-information storing unit 23 (step S14). As a resultof the search, when the record of the same combination is present (Yesat step S14), the token-circulation-order determining unit 24 registersSAs of all corresponding records in the token circulation table, whichis the token circulation order information, of thetoken-circulation-order-information storing unit 25 such that the SAsare connected to the MAC address of the ring management station Xregistered at step S13 (step S15).

Thereafter, the token-circulation-order determining unit 24 selects oneSA among the SAs (MAC addresses) of communication nodes registered inthe token circulation table (step S16). Subsequently, thetoken-circulation-order determining unit 24 searches in order throughthe network presence information to determine whether a record havingsame “SA information in the received network presence check frame” asthe selected SA is present, i.e., whether a lower-order communicationnode of the selected SA is present (step S17).

The token-circulation-order determining unit 24 determines whether acommunication node is present under a communication node correspondingto the selected SA (step S18). When a communication node is presentunder the selected SA (Yes at step S18), the token-circulation-orderdetermining unit 24 registers an SA of a record corresponding to thelower-order communication node in the token circulation table such thatthe SA is connected to the SA of the communication node (the slavestation) selected at step S16 (step S19).

Subsequently, the token-circulation-order determining unit 24 selectsone SA among the SAs registered at step S19 (step S20) and searches inorder through the network presence information to determine whether arecord having same “SA information in the received network presencecheck frame” as the selected SA is present, i.e., whether acommunication node under a communication node corresponding to theselected SA is present (step S21). Thereafter, thetoken-circulation-order determining unit 24 returns to step S18 andrepeatedly performs the processing at steps S18 to S21 until theprocessing reaches a lowest-order communication node in one branchselected out of branches branching in order from the ring managementstation X in a tree-like network structure.

When the processing reaches a lowest-order communication node of acertain branch in the tree-like network configuration, a lower-ordercommunication node is not present in the processing for searching for alower-order communication node of the selected SA at step S21.Therefore, when a lower-order communication node is not present in theselected SA at step S18 (No at step S18), the processing shifts.

In this case, the token-circulation-order determining unit 24 determineswhether another unsearched SA of the same level as an SA selected lastat step S16 or S20 is present (step S22). The level means, when SAs areconnected in a tree shape from the ring management station X, the numberof communication nodes in a path leading from the ring managementstation X to a certain SA.

When another unsearched SA at the same level as the SA selected last ispresent (Yes at step S22), the token-circulation-order determining unit24 selects another SA registered in the token circulation table of thesame level as the SA selected last (step S23). Subsequently, thetoken-circulation-order determining unit 24 searches in order throughthe network presence information to determine whether a record havingsame “SA information in the received network presence check frame” asthe selected SA is present, i.e., whether a lower-order communicationnode of the selected SA is present (step S24). Thereafter, thetoken-circulation-order determining unit 24 returns to step S18.

On the other hand, when another unsearched SA of the same level as theSA selected last is not present (No at step S22), thetoken-circulation-order determining unit 24 returns to an SA of a higherlevel nearest to the SA selected last (step S25). Thereafter, thetoken-circulation-order determining unit 24 determines whether the SA towhich the token-circulation-order determining unit 24 returns is an SAof a highest level, i.e., whether the SA is the MAC address of the ringmanagement station X (step S26). When the SA to which thetoken-circulation-order determining unit 24 returns is not the SA of thehighest level (No at step S26), the token-circulation-order determiningunit 24 determines whether an unsearched SA registered in the tokencirculation level of the same level as the SA to which thetoken-circulation-order determining unit 24 returns is present (stepS27).

When an unsearched SA registered in the token circulation table of thesame level as the SA to which the token-circulation-order determiningunit 24 returns is present (Yes at step S27), thetoken-circulation-order determining unit 24 selects another unsearchedSA registered in the token circulation table of the same level as the SAto which the token-circulation-order determining unit 24 returns (stepS28). Subsequently, the token-circulation-order determining unit 24searches in order through the network presence information to determinewhether a record having same “SA information in the received networkpresence check frame” as the selected SA is present, i.e., whether alower-order communication node of the selected SA is present (step S29).Thereafter, the token-circulation-order determining unit 24 returns tostep S18.

When an unsearched SA registered in the token circulation table of thesame level as the SA to which the token-circulation-order determiningunit 24 returns is not present at step S27 (No at step S27), thetoken-circulation-order determining unit 24 returns to step S25.

According to the execution of the processing explained above, allcommunication nodes connected to the port selected at step S12 of thering management station having the MAC address selected at step S11 areextracted in a tree shape.

When a record of a combination of “SA information in the receivednetwork presence check frame” and “port information of a station thattransmits the network presence check frame”, which is the same as thecombination of the selected MAC address and the selected port, is notpresent in the network presence information at step S14 (No at step S14)or when the SA to which the token-circulation-order determining unit 24returns is the highest-order level at step S26 (Yes at step S26), thetoken-circulation-order determining unit 24 determines whether there isan unsearched port of the ring management station X (step S30). Whenthere is an unsearched port (Yes at step S30), thetoken-circulation-order determining unit 24 returns to step S12. Theprocessing explained above is performed concerning the unsearched portof the ring management station X. When there is no unsearched port (Noat step S30), because the registration processing in the tokencirculation table is performed concerning all the communication nodes,the processing for determining a circulation order of a token ends.

According to the processing explained above, in the networkconfiguration in which the communication nodes are connected in a treeshape while branching from the ring management station X, when a pathleading from the ring management station X to a lowest-ordercommunication node immediately under the ring management station X isestablished, the token-circulation-order determining unit 24 returns toa communication node branching in a position closest to the lowest-ordercommunication node and establishes a path leading from the position toanother lowest-order communication node. The token-circulation-orderdetermining unit 24 repeats such processing in order and establishespaths leading to all the lowest-order communication node. Consequently,in the network including the communication nodes connected in a treeshape, the communication nodes can be connected by the method of drawingwith a single stroke of the brush. The number of communication nodesthat a token as a transmission right passes is minimized in a pathstarting from the ring management station X established in this way andreturning to the ring management station X.

Specific processing for determining circulation order of a token shownin FIG. 7 is explained with reference to an example in which thecirculation order is determined using the network presence informationshown in FIG. 6. FIGS. 8-1 to 8-9 are schematic diagrams of an exampleof a creating process for a token circulation table.

First, the ring management station X selects the MAC address “100” ofthe own station and the second port “port_X2” (steps S11 to S12). Asshown in FIG. 8-1, the ring management station X registers the selectedMAC address and port “100 (port_X2)” of the own station in the tokencirculation table (step S13). Thereafter, the ring management station Xperforms search to determine whether a record, “SA information in areceived network presence check frame” of which is “100” and “portinformation of a station that transmits the network presence checkframe” of which is “port_X2”, is present in the network presenceinformation shown in FIG. 6. As a result, the ring management station Xextracts a record 302.

Because an SA of this record 302 is “2 (the slave station B)”, the ringmanagement station X registers this SA “2” in the token circulationtable (step S15). As shown in FIG. 8-2, a state in which the SA “2” isregistered on the right side of the MAC address “100 (port_X2)” andconnected by an arrow extending from the MAC address “100 (port_X2)” tothe SA “2”. The direction of the arrow indicates a lower-order level.

Thereafter, the ring management station X selects this SA “2” in thetoken circulation table (step S16) and performs search to determinewhether a record, “SA information in a received network presence checkframe” of which is “2”, is present in the network presence informationshown in FIG. 6 (step S17). As a result, the ring management station Xextracts records 303 and 304. Because SAs of these records 303 and 304are respectively “3 (the slave station C)” and “4 (the slave stationD)”, as shown in FIG. 8-3, the SA “3” and the SA “4” are registered tobe arranged in parallel under the SA “2” of the token circulation table(step S19).

Subsequently, the ring management station X selects the SA “3” among theregistered SAs (step S20) and performs search to determine whether arecord, “SA information in a received network presence check frame” ofwhich is “3”, is present in the network presence information shown inFIG. 6 (step S21). However, because a relevant record is not present inthe network presence information shown in FIG. 6, the record is notextracted (No at step S18). In other words, a communication node is notpresent under the SA “3”. As shown in FIG. 8-4, “none” is describedunder the SA “3” of the token circulation table for convenience.

Subsequently, the ring management station X selects the SA “4” asanother unsearched SA of the same level as the SA “3” (steps S22 toS23). The ring management station X performs search to determine whethera record, “SA information in a received network presence check frame” ofwhich is “4”, is present in the network presence information shown inFIG. 6 (step S24). As a result, the ring management station X extracts arecord 305. Because an SA of this record 305 is “5 (the slave stationE)”, as shown in FIG. 8-5, the ring management station X registers thisSA “5” under the SA “4” (step S19).

Thereafter, the ring management station X selects the registered SA “5”(step S20) and performs search to determine whether a record, “SAinformation in a received network presence check frame” of which is “5”,is present in the network presence information shown in FIG. 6 (stepS21). However, because a relevant record is not present in the networkpresence information shown in FIG. 6, the record is not extracted (No atstep S18). In other words, no node is present under the SA “5”. As shownin FIG. 8-6, “none” is described under the SA “5” of the tokencirculation table for convenience.

Because another unsearched SA of the same level as the SA “5” is notpresent (No at step S22), the ring management station X returns to theSA “4”, which is the nearest SA of the higher level (step S25). Becausethis SA “4” is not the highest level (No at step S26) and anotherunsearched SA registered in the token circulation table at the samelevel as the SA “4” is not present (No at step S27), the ring managementstation X returns to the SA “2”, which is the higher level nearest tothe SA “4” (step S25).

However, because the SA “2” is not the highest level (No at step S26)and another unsearched SA registered in the token circulation table atthe same level as the SA “2” is not present (No at step S27), the ringmanagement station X returns to the MAC address “100”, which is thehigher level nearest to the SA “2” (step S25).

Because the MAC address “100” is the highest level (Yes at step S26),the ring management station X checks whether an unsearched port ispresent in the ring management station X (step S30). Because the firstport “port_X1” is unsearched, the ring management station X selects thefirst port “port_X1” (step S12). As shown in FIG. 8-7, the ringmanagement station X registers the MAC address and port “100 (port_X1)”of the own station in the token circulation table (step S13). The MACaddress and first port “100 (port_X1)” of the ring management station Xis arranged in parallel to the MAC address and second port “100(port_X2)”. Thereafter, the ring management station X performs search todetermine whether a record, “SA information in a received networkpresence check frame” of which is “100” and “port information of astation that transmits the network presence check frame” of which is“port_X1”, is present in the network presence information shown in FIG.6. As a result, the ring management station X extracts the record 301.

Because an SA of this record 301 is “1 (the slave station A)”, the ringmanagement station X registers this SA “1” in the token circulationtable (step S15). As shown in FIG. 8-8, the SA “1” is registered underthe MAC address “100 (port_X1)”.

Thereafter, the ring management station X selects this SA “1” (step S16)and performs search to determine whether a record, “SA information in areceived network presence check frame” of which is “1”, is present inthe network presence information shown in FIG. 6 (step S17). However,because a relevant record is not present in the network presenceinformation shown in FIG. 6, the record is not extracted (No at stepS18). In other words, a node is not present under the SA “1”. As shownin FIG. 8-9, “none” is described under the SA “1” of the tokencirculation table for convenience.

Because another unsearched SA of the same level as the SA “1” is notpresent (No at step S22), the ring management station X returns to theMAC address “100”, which is a nearest SA of a higher level (step S25).Because the MAC address “100” is the highest level (Yes at step S26),the ring management station X checks whether an unsearched port ispresent in the ring management station X (step S30). Because search isalready performed concerning all the ports as explained above (No atstep S30), the processing for determining circulation order of a tokenends. As a result, a token circulation table shown in FIG. 8-9 isobtained. From FIG. 8-9, for example, token circulation order indicatedby (A) below is obtained.

Ring management station X→slave station B→slave station C→slave stationD→slave station E→slave station A→ring management station X (A)

In FIG. 8-9, a token frame can be circulated from either the first portX1 or the second port X2 of the ring management station X. Therefore,token circulation order indicated by (B) below can be adopted.

Ring management station X→slave station A→slave station B→slave stationC→slave station D→slave station E→ring management station X (B)

Further, in the tree structure connected to the first port X1 of thering management station X, whichever of branching portions can be setfirst. Therefore, token circulation order indicated by (C) and tokencalculation order indicated by (D) can be adopted.

Ring management station X→slave station B→slave station D→slave stationE→slave station C→slave station A→ring management station X (C)

Ring management station X→slave station A→slave station B→slave stationD→slave station E→slave station C→ring management station X (D)

The token circulation order (A) to the token circulation order (D) canbe drawn with a single stroke of the brush. In the path starting fromthe ring management station X and returning to the ring managementstation X, the number of communication nodes that a token frame passes(the number of times the token frame is propagated through thetransmission lines among the communication nodes) is nine, which is theminimum number. In this way, the number of transmission lines throughwhich the token frame is propagated while being circulated once isminimized, whereby circulation efficiency of the token frame is improvedand wasteful communication can be suppressed. This contributes to energysaving.

The token circulation table created as explained above is stored in thetoken-circulation-order-information storing unit 25 as token circulationorder information. Processing for establishing a logical ring in thecommunication system is performed based on this determined tokencirculation order.

Thereafter, the setup processing unit 26 of the ring management stationX generates, using the token circulation table stored in thetoken-circulation-order-information storing unit 25, a setup frame fornotifying circulation information of the transmission right of thecommunication nodes.

FIG. 5-7 is a schematic diagram of an example of a setup frametransmitted from the ring management station X to the slave stations.For example, in a setup frame transmitted to the slave station B, asindicated by “SetupFrame(X→B)”, the MAC address “2” of the slave stationB, which is a setting target, is set in a DA, the MAC address “100” ofthe own station is set in an SA, “Setup” is stored in frame typeinformation, and the MAC address “3” of the slave station C, which is acommunication node to which a token frame is transmitted after the slavestation B, is set in token frame destination MAC address settinginformation. Setup frames transmitted to the other slave stations A andC to E are generated in the same manner.

As shown in FIG. 4-8, the setup processing unit 26 of the ringmanagement station X transmits the created setup frame to the slavestations A to E via the first and second ports X1 and X2. The slavestation B repeats the setup frame addressed to the slave stations C, D,and E. The slave station D repeats the setup frame addressed to theslave station E.

When the slave stations A to E receive the setup frame from the ringmanagement station X, the control-frame response units 61 of the slavestations A to E read out a MAC address stored in “token framedestination MAC address setting information” in the setup frame andstore the MAC address in the token-circulation-destination-informationstoring units 62 as token circulation destination information. In thisway, in the first embodiment, the slave stations A to E store, as thetoken circulation destination information, only a MAC address of acommunication node to which a token should be transmitted next.Therefore, information for circulating a token frame is minimizedcompared with the time when all the slave stations A to E store, incommon, the token circulation table that is information concerningcirculation order of all token frames.

After storing the token circulation destination information in thetoken-circulation-destination-information storing units 62, thecontrol-frame response units 61 of the slave stations A to E generatesetup response frames (in the figure, represented as SetupACK), whichare responses to the setup frame. As shown in FIG. 4-9, thecontrol-frame response units 61 transmit the setup response frames tothe ring management station X. The slave station D repeats the setupresponse frame from the slave station E. The slave station B repeats thesetup response frames from the slave stations C, D, and E.

FIG. 5-8 is a schematic diagram of an example of setup response framestransmitted from the slave stations. For example, in a setup responseframe transmitted from the slave station A to the ring managementstation X, as indicated by “SetupACKFrame(A→X)”, the MAC address “100”of the ring management station X is set in a DA, the MAC address “1” ofthe own station is set in an SA, and “SetupACK” is stored in frame typeinformation. Setup response frames transmitted from the other slavestations B to E are generated in the same manner.

After the control-frame response units 61 of the slave stations A to Etransmit the generated setup response frames to the ring managementstation X, the token-frame processing units 63 start observation of atoken frame flowing on the transmission lines.

Thereafter, the ring management station X receives the setup responseframes from the slave stations A to E and recognizes that the setupframe generated by the ring management station X normally reaches theslave stations A to E. When the setup response frames reach the ringmanagement station X from not all of the slave stations A to E within apredetermined time, the setup processing unit 26 of the ring managementstation X transmits the setup frame to the slave stations from which thesetup response frames are not received or all the slave stations again.Alternatively, the setup processing unit 26 determines that the powersupplies for the slave stations are turned off during setup and performsthe processing from the beginning in FIG. 4-1. Further, when the setupprocessing unit 26 is caused to operate in this way, the setupprocessing unit 26 starts the timer 21 during the transmission of thesetup frame and counts the predetermined time. It is assumed that thesetup response frames are received from all the slave stations A to E.

Subsequently, when the token-frame processing unit 27 of the ringmanagement station X recognizes that the setup response frames arereceived from all the slave stations A to E in the communication system,the token-frame processing unit 27 recognizes from the token circulationorder information that the own station has the transmission right. Thedata-frame-communication processing unit 28 transmits a data frame tothe slave stations A to E.

Thereafter, the token-frame processing unit 27 of the ring managementstation X generates the token frame 280 according to the tokencirculation table of the token-circulation-order-information storingunit 25. The token-frame processing unit 27 generates, according to FIG.8-9, a token frame for giving the transmission right to the slavestation B.

FIG. 5-9 is a schematic diagram of an example of token frames generatedby the communication nodes. As indicated by “TokenFrame(X→B)” of thisfigure, a broadcast address is set in a DA, the MAC address “100” of theown station is set in an SA, “token” is set in frame type information,and the MAC address “2” of the slave station B, which is a communicationnode that should acquire the next transmission right, is stored in MACaddress information for token-addressed-to-own-station determination.

Thereafter, as shown in FIG. 4-10, the token-frame processing unit 27 ofthe ring management station X transmits the generated token frame fromthe first and second ports X1 and X2. Because the token frame istransmitted in broadcast, the token frame is transmitted to all theslave stations A to E in the communication system. The slave stations Ato E have a function of repeating the token frame when the token frameis received. In this figure, for convenience of explanation, the tokenframe is drawn to be simultaneously transmitted to all the slavestations. However, actually, the token frame is repeated to the slavestations C and D by the slave station B and repeated to the slavestation E by the slave station D.

When the token-frame processing units 63 of the slave stations A to Ereceive the token frame, the token-frame processing units 63 compare aMAC address stored in “MAC address information fortoken-addressed-to-own-station determination” in a data area of thetoken frame and MAC addresses of the own stations. The token-frameprocessing units 63 of the slave stations A and C to E determine thatthe transmission right cannot be acquired because a MAC address (2)stored in the “MAC address information fortoken-addressed-to-own-station determination” and the MAC addresses ofthe own stations (1 and 3 to 5) do not coincide with each other. On theother hand, the token-frame processing unit 63 of the slave station Bdetermines that the transmission right is acquired because the MACaddress (2) stored in the “MAC address information fortoken-addressed-to-own-station determination” and the MAC address (2) ofthe own station coincide with each other. The data-frame-communicationprocessing unit 64 of the slave station B transmits data to apredetermined destination (e.g., the ring management station X).Thereafter, the token-frame processing unit 63 performs preparation fortransmitting the token frame to pass the transmission right to the nextcommunication node.

The token-frame processing unit 63 of the slave station B acquires tokencirculation destination information, i.e., a MAC address “3” of theslave station C, which obtains the transmission right after the ownstation, from the token-circulation-destination-information storing unit62. As indicated by “TokenFrame(B→C)” of FIG. 5-9, the token-frameprocessing unit 63 sets acquired “3” in the “MAC address fortoken-addressed-to-own-station determination information” of the dataarea of the token frame and sets the MAC address “2” of the own stationin the SA.

Thereafter, as shown in FIG. 4-11, the token-frame processing unit 63 ofthe slave station B issues a token frame from all the output ports (thefirst and second ports B1 and B2). Because the token frame istransmitted in broadcast, the token frame is transmitted to all thecommunication nodes in the communication system. The token-frameprocessing units 27 and 63 of the ring management station X and theslave stations A and C to E have a function of repeating the token framewhen the token frame is received.

The ring management station X and the slave stations A, D, and E comparethe MAC address (3) stored in “MAC address information fortoken-addressed-to-own-station determination” in a data area of thereceived token frame and MAC addresses of the own stations (100, 1, 4,and 5). Because the MAC address (3) and the MAC addresses of the ownstations do not coincide with each other, the ring management station Xand the slave stations A, D, and E determine that the transmission rightcannot be acquired. On the other hand, the slave station C determinesthat the transmission right is acquired because the MAC address (3) anda MAC address of the own station coincide with each other. Aftertransmitting data to a predetermined destination, the slave station Cperforms preparation for transmitting the token frame to pass thetransmission right to the next communication node.

Thereafter, the token frame is transmitted to the communication nodes inorder in the same manner as the processing explained above according tothe token circulation table shown in FIG. 8-9. Specifically, the slavestation C generates a token frame indicated by “TokenFrame(C→D)” of FIG.5-9 and transmits the token frame in the communication system inbroadcast as shown in FIG. 4-12. Consequently, the slave station Dacquires the transmission right. The slave station D generates a tokenframe indicated by “TokenFrame(D→E)” of FIG. 5-9 and transmits the tokenframe in the communication system in broadcast as shown in FIG. 4-13.The slave station E acquires the transmission right. The slave station Egenerates a token frame indicated by “TokenFrame(E→A)” of FIG. 5-9 andtransmits the token frame in the communication system in broadcast asshown in FIG. 4-14. The slave station A acquires the transmission right.The slave station A generates a token frame indicated by “TokenFrame(A→X)” of FIG. 5-9 and transmits the token frame in the communicationsystem in broadcast as shown in FIG. 4-15. The ring management station Xacquires the transmission right.

Because the token-frame processing unit 63 of the slave station Arecognizes that a communication node is not connected to the first portA1 via a transmission line, the token-frame processing unit 63 issues atoken frame from the second port A2. Because the token-frame processingunits 63 of the slave stations C and E recognize that communicationnodes are not connected to the second ports C2 and E2 via transmissionlines, the token-frame processing units 63 issue token frames from thefirst ports C1 and E1.

Such processing is repeatedly performed. A token frame is circulatedamong the communication nodes in the network of the same segmentaccording to token circulation order. A communication node that receivesthe token frame obtains the transmission right for data and transmitsthe data to a desired destination. As explained above, the establishingmethod for a logical ring and the communication method employing a tokenare performed.

According to the first embodiment, in the communication system in whichthe communication nodes are connected in a star shape or a line shape bythe Ethernet, order for circulating a token frame for obtaining thetransmission right is determined by the method of drawing with a singlestroke of the brush. Therefore, it is possible to minimize the number ofcommunication nodes (or the number of transmission lines) that the tokenframe passes. As a result, there is an effect that it is possible toefficiently circulate the token frame.

After establishing a logical ring, the ring management station Xnotifies each of the communication nodes (the slave stations A to E) inthe communication system of only a MAC address of a communication nodethat acquires the transmission right after the communication node.Therefore, the communication nodes have only minimum informationnecessary for circulating the token frame. As a result, compared withthe time when the slave stations A to E store all circulation orders ofthe token frame in the communication system, it is possible to increasethe speed of processing for circulating the token frame and it ispossible to reduce the capacity of a memory for storing tokencirculation destination information. Therefore, it is possible to reducemanufacturing cost for the apparatus.

Further, after establishing the logical ring, when responses to a setupframe transmitted to the communication nodes (the slave stations A to E)in the communication system are returned from all the communicationnodes, the ring management station X issues a token frame. Therefore,there is also an effect that is it possible to recognize that thelogical ring is lost because of a reason such as power-off of thecommunication nodes (the slave stations A to E) after the logical ringis established and before the token frame is issued.

Second Embodiment

In a second embodiment, a method of establishing a logical ring when acommunication node is added anew in a communication system thatcirculates a token frame and performs transmission of data (a frame) isexplained.

The ring management station X in the second embodiment basically has asame configuration as that shown in FIG. 2-1 in the first embodiment.Therefore, explanation of the configuration is omitted. However, thetoken-frame processing unit 27 further has a function of counting thenumber of times the own station acquires a token frame and, when thenumber of times reaches a predetermined number of times set in advance,causing the network-presence-check processing unit 22 to execute networkpresence check processing.

The setup processing unit 26 further has a function of creating a setupframe and transmitting the setup frame to only a communication node (aslave station) for which token circulation order stored in thetoken-circulation-order-information storing unit 25 changes before andafter the network presence check processing.

Slave stations in the second embodiment basically have a sameconfiguration as that shown in FIG. 2-2 in the first embodiment.Therefore, explanation of the configuration is omitted. However, thecontrol-frame response unit 61 further has a function of, when tokencirculation destination information is stored in thetoken-circulation-destination-information storing unit 62, not returninga network presence acknowledge frame when the network presence checkframe is received from the ring management station X.

The method of establishing a logical ring according to the secondembodiment is explained below. FIGS. 9-1 to 9-11 are diagrams of anexample of a processing procedure of the method of establishing alogical ring when a slave station is added anew to the communicationsystem. In the example explained below, a slave station F is connectedto the second port C2 of the slave station C in the networkconfiguration shown in FIG. 1 in the first embodiment.

First, when the ring management station X starts issuance of a tokenframe in FIG. 4-10 in the first embodiment, the token-frame processingunit 27 starts counting of the number of times a token (a transmissionright) is acquired. As explained in the first embodiment, the tokenframe is circulated in order among the communication nodes in thecommunication system. In FIG. 9-1, it is assumed that the ringmanagement station X acquires the token a predetermined number of times(e.g., ten times).

The token-frame processing unit 27 of the ring management station Xnotifies the network-presence-check processing unit 22 that the tokenframe is acquired the predetermined number of times. As shown in FIG.9-2, the network-presence-check processing unit 22 generates a networkpresence check frame and transmits the network presence check frame tothe communication nodes (the slave stations A to E) in the communicationsystem in broadcast as explained in the first embodiment. At the sametime, the network-presence-check processing unit 22 starts the timer 21set to a predetermined time to wait for the network presence acknowledgeframe. As explained in the first embodiment, when the slave stations Band D receive the network presence check frame, the slave stations B andD reconfigure and transmit the network presence check frame.

The slave stations A to E in the communication system receives thenetwork presence check frame broadcasted from the ring managementstation X. However, in a state in which this network presence checkframe is received, the token circulation destination information isstored in the token-circulation-destination-information storing unit 62of the slave stations A to E. Therefore, as shown in FIG. 9-3, thecontrol-frame response units 61 of the slave stations A to E do notrespond to the network presence check frame.

At a point when the ring management station X transmits the networkpresence check frame, a new communication node is not connected to thecommunication system. Therefore, while the timer 21 set by thenetwork-presence-check processing unit 22 of the ring management stationX counts the predetermined time, the ring management station X does notreceive the network presence acknowledge frame.

When the timer 21 times out, because there is no change in the networkpresence information stored in the network-presence-information storingunit 23, the token-circulation-order determining unit 24 of the ringmanagement station X does not perform processing for determining tokencirculation order. Because there is no change in the token circulationorder as well, the setup processing unit 26 does not performtransmission processing for a setup frame. The token-frame processingunit 27 notifies the data-frame-communication processing unit 28 thatthe transmission right is present in the own station. Thedata-frame-communication processing unit 28 performs processing fortransmitting data of the own station to the other communication nodes(the slave stations A to E) in the communication system. At this point,the token-frame processing unit 27 resets the number of times ofacquisition of the token frame and starts counting of the number oftimes of acquisition of the token frame again.

Subsequently, as shown in FIG. 9-4, the token-frame processing unit 27generates a token frame according to the token circulation orderinformation to pass the transmission right to the slave station B andtransmits the token frame from the first and second ports X1 and X2 inbroadcast. Thereafter, as explained in the first embodiment, thetransmission right is given in order to the slave stations A to Eaccording to the token circulation destination information and is givento the ring management station X in the communication system accordingto the token circulation order information. Transmission processing fordata (a frame) is performed from the communication nodes that acquirethe transmission right.

At this point, as shown in FIG. 9-5, it is assumed that the slavestation F is connected anew to the second port C2 of the slave station Cvia the cable 102. A MAC address of this slave station F is “6”. Likethe other slave stations A to E, the slave station F includes first andsecond ports F1 and F2.

When a power supply is turned on, this slave station F changes to astate of waiting for a network presence check frame from the ringmanagement station X. Information concerning a token circulationdestination that is a next transmission destination of a token frame isnot stored in the token-circulation-destination-information storing unit62. Therefore, in this state, transmission and reception of data in thecommunication system is not performed. In the communication system, atoken frame and data (a frame) are transmitted and received between thering management station X and the slave stations A to E.

Thereafter, when the token frame is circulated and, as shown in FIG.9-6, the token frame is acquired a predetermined number of times (e.g.,ten times) in the ring management station X, the token-frame processingunit 27 of the ring management station X notifies thenetwork-presence-check processing unit 22 that the token frame isacquired the predetermined number of times. As shown in FIG. 9-7, thenetwork-presence-check processing unit 22 generates a network presencecheck frame and transmits the network presence check frame to thecommunication nodes (the slave stations A to F) in the communicationsystem in broadcast. At the same time, the network-presence-checkprocessing unit 22 starts the timer 21 set to the predetermined time towait for the network presence acknowledge frame.

As explained in FIG. 9-3, even if the slave stations A to E in thecommunication system receive the network presence check frame from thering management station X, the token circulation destination informationis stored in the token-circulation-destination-information storing units62 of the slave stations A to E at the time of the reception of thenetwork presence check frame. Therefore, as shown in FIG. 9-8, thecontrol-frame response units 61 of the slave stations A to E do notrespond to the network presence check frame.

On the other hand, the token circulation destination information is notstored in the token-circulation-destination-information storing unit 62of the slave station F connected anew. Therefore, when the control-frameresponse unit 61 receives the network presence check frame from the ringmanagement station X, the control-frame response unit 61 generates anetwork presence acknowledge frame, which is a response to the networkpresence check frame. As shown in FIG. 9-8, the control-frame responseunit 61 transmits the generated network presence acknowledge frame (inthe figure, represented as TestDataACKFrame (F1→X)) from the first portF1 to the ring management station X. In FIG. 9-8, the slave stations Cand B relay the network presence acknowledge frame transmitted by theslave station F.

FIG. 10 is a schematic diagram of an example of the network presenceacknowledge frame transmitted from the slave station F. In the networkpresence acknowledge frame transmitted from the first port F1 of theslave station F, the MAC address “100” of the ring management station Xis set in a DA, the MAC address “6” of the own station is set in an SA,“TestDataACK” is stored in frame type information, and “3” and “port_C2”are respectively set in SA information of the received network presencecheck frame and port information of a station that transmits the networkpresence check frame.

The network presence acknowledge frame transmitted from the slavestation F is received by the ring management station X within a startingperiod of the timer 21. The network-presence-check processing unit 22 ofthe ring management station X extracts, from the network presenceacknowledge frame received from the slave station F, the SA, the SAinformation in the received network presence check frame, and the portinformation of the station that transmits the network presence checkframe and stores the SA, the SA information, and the port information inthe network-presence-information storing unit 23 as network presenceinformation. FIG. 11 is a diagram of an example of network presenceinformation after the network presence acknowledge frame is receivedfrom the slave station F. In this network presence information, comparedwith the network presence information shown in FIG. 6 in the firstembodiment, a record 306 concerning the slave station F is added.

When the timer 21 times out, the token-circulation-order determiningunit 24 of the ring management station X detects a change in the networkpresence information stored in the network-presence-information storingunit 23 and performs, using the network presence information, processingfor determining token circulation order according to a procedure same asthat explained in the first embodiment, i.e., reconfiguration processingfor a logical ring. As a result, it is assumed that new tokencirculation order is as indicated by (E) below.

Ring management station X→slave station B→slave station C→slave stationF→slave station D→slave station E→slave station A→ring managementstation X (E)

Thereafter, the setup processing unit 26 of the ring management stationX compares the token circulation order before the change and the newtoken circulation order and extracts the communication nodes for whichthe token circulation destination information is different from thatbefore the change. When the token circulation order (A) explained in thefirst embodiment and the new token circulation order (E) are compared,because the slave station F is connected anew, token circulationdestination information is set anew in the slave station F and the tokencirculation destination information of the slave station C immediatelybefore the slave station F is changed.

Therefore, the setup processing unit 26 creates setup frames for theslave stations C and F. FIG. 12 is a schematic diagram of an example ofsetup frames transmitted from the ring management station X to the slavestations C and F. In the setup frame transmitted to the slave station C,as indicated by “SetupFrame (X→C)”, the MAC address “3” of the slavestation C, which is a setting target, is set in a DA, the MAC address“100” of the own station is set in an SA, “Setup” is stored in frametype information, and the MAC address “6” of the slave station F, whichis a communication node to which the token frame is transmitted afterthe slave station C, is set in token frame destination MAC addresssetting information.

In the setup frame transmitted to the slave station F, as indicated by“SetupFrame (X→F)”, the MAC address “6” of the slave station F, which isa setting target, is set in the DA, the MAC address “100” of the ownstation is set in the SA, “Setup” is stored in the frame typeinformation, and the MAC address “4” of the slave station D, which is acommunication node to which the token frame is transmitted after theslave station F, is set in the token frame destination MAC addresssetting information.

As shown in FIG. 9-9, the setup processing unit 26 of the ringmanagement station X transmits the created setup frames respectively tothe slave stations C and F. The slave station B repeats the setup framesaddressed to the slave stations C and F. The slave station C repeats thesetup frame addressed to the slave station F.

When the slave stations C and F receive the setup frames from the ringmanagement station X, the control-frame response units 61 of the slavestations C and F read out MAC addresses stored in the “token framedestination MAC address setting information” in the setup frames andstore the MAC addresses in the token-circulation-destination-informationstoring units 62 as token circulation destination information.

After storing the token circulation destination information in thetoken-circulation-destination-information storing unit 62, thecontrol-frame response units 61 of the slave stations C and F generatesetup response frames (in the figure, represented as SetupACK), whichare responses to the setup frames. In the setup response frame generatedby the slave station C, as shown in FIG. 5-8, the MAC address of the ownstation is set in an SA and “SetupACK” is stored in frame typeinformation in a data area. The setup response frame transmitted fromthe slave station F is generated in the same manner.

Subsequently, as shown in FIG. 9-10, the control-frame response units 61of the slave stations C and F transmit the setup response frames to thering management station X. The token-frame processing unit 27 startsobservation of the token frame flowing on the transmission lines.

When the token-frame processing unit 27 of the ring management station Xreceives the setup response frames from the slave stations C and F, thetoken-frame processing unit 27 notifies the data-frame-communicationprocessing unit 28 that the transmission right is present in the ownstation. The data-frame-communication processing unit 28 performsprocessing for transmitting data of the own station to the othercommunication nodes (slave stations A to F) in the communication system.At this point, the token-frame processing unit 27 resets the number oftimes of acquisition of the token frame and starts the counting of thenumber of times of acquisition of the token frame again.

Subsequently, as shown in FIG. 9-11, the token-frame processing unit 27of the ring management station X generates a token frame according tothe token circulation order information to pass the transmission rightto the slave station B and transmits the token frame from the first andsecond ports X1 and X2 in broadcast. Thereafter, as explained in thefirst embodiment, in the slave stations A to F, according to the tokencirculation destination information and, in the ring management stationX, according to the token circulation table (the token circulation orderinformation), the transmission right is given in order to thecommunication nodes in the communication system to which the slavestation F is added. Transmission processing for data (a frame) isperformed from the communication nodes that obtain the transmissionright.

According to the second embodiment, even when a communication node (aslave station) is connected to the communication system anew while thetoken frame is circulated, the ring management station X performs thelogical ring reconfiguration processing when the ring management stationX acquires the token frame the predetermined number of times. As aresult, the ring management station X can recognize the newcommunication node as a station included in the communication system andperform transmission and reception of data including the newcommunication node.

The slave stations A to F are configured, when the token circulationdestination information is already stored in thetoken-circulation-destination-information storing unit 62, not torespond to the network presence check frame periodically transmittedfrom the ring management station X. Therefore, a frame having sameinformation as information already accumulated in the ring managementstation is not uselessly transmitted.

Further, the ring management station X performs the reconfigurationprocessing for a logical ring only when there is a change in the networkpresence information, determines token circulation order informationagain, compares the token circulation order information with the tokencirculation order before the redetermination, and notifies only thecommunication node connected anew and the communication node connectedat the pre-stage of the communication node of token circulationdestination information, which is a destination of a token frametransmitted next. As a result, it is possible to hold down a use band ofthe transmission lines and perform an efficient change compared with thetime when the token circulation destination information is notified toall the communication nodes in the communication system.

Third Embodiment

In the second embodiment, a communication node connected anew to thecommunication system is recognized and the reconfiguration processingfor a logical ring is performed while the token frame is circulated.However, in the network presence check processing for determiningwhether periodically-performed reconfiguration of a logical ring isnecessary, it is likely that a large number of communication nodes arerecognized at a time. In general, real-time properties of communicationare requested in an FA network. However, when a large number ofcommunication nodes are connected at a time, a long time is required forthe reconfiguration processing for a logical ring in the ring managementstation. As a result, it is likely that the real-time properties arespoiled. Therefore, in a third embodiment, a method of performing, evenwhen a large number of communication nodes are substantiallysimultaneously connected to a communication system at a time,reconfiguration processing for a logical ring including a newcommunication node without spoiling real-time properties in thecommunication system in which transmission and reception of data isalready performed is explained.

The configuration of a ring management station in the third embodimentis basically the same as that explained in the first and secondembodiments. Therefore, explanation of the same components is omitted.However, the third embodiment is different from the first and secondembodiments in that the token-circulation-order determining unit 24extracts records of a maximum number of communication nodes to be addedset in advance from records stored temporally earlier (older) amongrecords stored anew in network presence information of thenetwork-presence-information storing unit 23 in network presence checkprocessing performed in the intervals of communication performed using atoken frame and performs determination processing for token circulationorder.

The maximum number of communication nodes to be added set in thetoken-circulation-order determining unit 24 is determined by calculatingin advance the number of communication nodes in which time required forthe reconfiguration processing for a logical ring performed in theintervals of communication performed using a token frame does not spoilreal-time properties of data transmission and reception in thecommunication system. In an example explained below, the maximum numberof communication nodes to be added is set to five.

According to the determination processing for token circulation order,network presence information is stored in thenetwork-presence-information storing unit 23 in reception order ofnetwork presence acknowledge frames. Information indicating whether acommunication node is a communication node subjected to thereconfiguration processing for a logical ring is further stored. FIG. 13is a diagram of an example of the structure of network presenceinformation according to the third embodiment. In this network presenceinformation, as explained with reference to FIG. 6, besides the items“SA”, “SA information in a received network presence check frame”, and“port information of a station that transmits the network presence checkframe”, an item “end of configuration processing for a logical ring”indicating whether a record is a record used during the configurationprocessing for a logical ring is added. When a flag is set in this itemof “end of configuration processing for a logical ring” (in FIG. 13, acircle is set), this indicates that this record is already used for thereconfiguration processing for a logical ring. In other words, thismeans that a record in which a flag is not set is not used for thereconfiguration processing for a logical ring.

In FIG. 13, it is assumed that record concerning communication nodes,SAs of which are 21 to 30, are records added anew in the networkpresence check processing performed in the intervals of thecommunication performed using a token frame and the records areregistered in order of arrival of network presence acknowledge frames tothe ring management station X. The network presence acknowledge framesreach the ring management station X earlier from communication nodescloser to the ring management station X.

The configuration of a slave station in the third embodiment is the sameas that explained in the first and second embodiments. Therefore,explanation of the configuration is omitted.

The reconfiguration processing for a logical ring according to the thirdembodiment is explained below. FIG. 14 is a flowchart for explaining anexample of the reconfiguration processing for a logical ring by the ringmanagement station. As shown in FIG. 9-11, the communication systemincludes the ring management station and the slave stations A to F.

First, the ring management station X receives a token frame in which MACaddress information for token-addressed-to-own-station determination isset to the own station and acquires a token (step S51). Subsequently,the token-frame processing unit 27 of the ring management station Xcounts the number of times of acquisition of the token (step S52) anddetermines whether the number of times of acquisition of the tokenreaches a predetermined number of times (step S53). When the number ofacquisition of the token does not reach the predetermined number oftimes (No at step S53), after transmitting a data frame to the slavestations A to F, the token-frame processing unit 27 generates a tokenframe in which a communication node at the next circulation destinationis set, transmits the token frame to the communication nodes in thecommunication system in broadcast (step S62), and returns to step S51.

When the number of times of acquisition of the token reaches thepredetermined number of times (Yes at step S53), thetoken-circulation-order determining unit 24 acquires a record alreadyused for the logical ring configuration processing from the networkpresence information of the network-presence-information storing unit 23(step S54). For example, in the case of FIG. 13, thetoken-circulation-order determining unit 24 acquires the records 301 to306 in which the “end of configuration processing for a logical ring”flag is set.

Thereafter, the token-circulation-order determining unit 24 determineswhether the number of records registered anew in the network presenceinformation after the (re)configuration processing for a logical ring isperformed is larger than the set maximum number of communication nodesto be added (step S55). When the number of records registered anew inthe network presence information is equal to or smaller than the setmaximum number of communication nodes to be added (No at step S55), thetoken-circulation-order determining unit 24 acquires all the recordsregistered anew, i.e., all the records not used for the logical ringconfiguration processing from the network presence information (stepS56).

On the other hand, when the number of records registered anew in thenetwork presence information is larger than the set maximum number ofcommunication nodes to be added (Yes at step S55), thetoken-circulation-order determining unit 24 acquires, from the networkpresence information, in order of registration in the network presenceinformation, records of the maximum number of communication nodes to beadded among the records not used for the logical ring configurationprocessing. For example, in the case of FIG. 13, thetoken-circulation-order determining unit 24 acquires five records 307among the records in which the “end of configuration processing for alogical ring” flag is not set. As a result, the remaining records 308are not used for the logical ring configuration processing.

Thereafter, the token-circulation-order determining unit 24 sets the“end of configuration processing for a logical ring” flag in the recordsacquired at step S56 or step S57 in the network presence information(step S58). For example, the token-circulation-order determining unit 24sets the “end of configuration processing for a logical ring” flag inthe records 307 shown in FIG. 13.

Subsequently, the token-circulation-order determining unit 24 performsthe token circulation order determination processing explained in thefirst embodiment using the acquired records (step S59). The setupprocessing unit 26 performs, according to the determined tokencirculation order information, setup processing for notifying the slavestations, which need notification of a token circulation destination, ofthe token circulation destination information (step S60). In this setupprocessing, a setup frame is not notified to the communication nodes(slave stations) registered in the network presence information but notused for the reconfiguration processing for a logical ring(determination processing for token circulation order). Thereafter, thetoken-frame processing unit 27 resets the number of times the token isacquired (step S61). The data-frame-communication processing unit 28transmits a data frame to the slave stations. Subsequently, thetoken-frame processing unit 63 generates a token frame in which acommunication node at the next circulation destination is set andtransmits the token frame to the communication node in the communicationsystem in broadcast (step S62). The ring management station X returns tostep S51 and the processing explained above is repeatedly performed.

As a result, for example, in the example shown in FIG. 13, when thenumber of times the token is acquired reaches the predetermined numberof times, the ring management station X performs the reconfigurationprocessing for a logical ring using the records 301 to 307 alreadysubjected to the logical ring configuration processing and the record308 registered in the network presence information last time but notused for the logical ring configuration processing.

According to the third embodiment, in the network presence checkprocessing for determining whether the periodically-performedreconfiguration of a logical ring is necessary, even when a large numberof communication nodes are recognized at a time, the configurationprocessing for a logical ring (the token circulation order determinationprocessing and the setup processing) using records in the networkpresence information of the communication nodes already subjected to thelogical ring configuration processing and the communication nodes in arange of the maximum number of communication nodes to be added amongcommunication nodes added anew. Consequently, there is an effect that itis possible to prevent time required for the logical ring configurationprocessing from spoiling real-time properties in transmission andreception of data of the communication system.

Fourth Embodiment

During transmission and reception of data performed using a token frame,in some case, a token frame or a data frame disappears because of theinfluence of external noise or the like. In a system in the past thatperforms communication using a token frame, when disappearance of aframe occurs, a management station reissues a token frame. However, thistoken frame is set to start from the management station. Therefore,communication nodes having the transmission right after a communicationnode to which the disappeared frame is transmitted cannot obtain thetransmission right for one transmission. The management station cannotreceive data equally from the communication nodes. In such a system inwhich disappearance of a frame frequently occurs, there is a problem inthat deviation occurs in transmission of data from the communicationnodes to the management station and affects accuracy of an arithmeticoperation, for example, when a feedback arithmetic operation isperformed using data from the communication nodes. In a fourthembodiment, a communication system is explained that can equally performdata transmission to the management station from the communication nodeseven when a frame disappears.

FIG. 15 is a schematic block diagram of an example of a functionalconfiguration of the ring management station according to the fourthembodiment. In the configuration shown in FIG. 2-1 in the firstembodiment, the ring management station X further includes a networkmonitoring unit 29 and a frame storing unit 30 in a communicationprocessing unit 20A.

When a frame flowing in a network is input to the first port 11-1 or thesecond port 11-2, the network monitoring unit 29 stores the frame in theframe storing unit 30, starts the timer 21, and monitors the frameflowing in the network. When a frame is input to the first port 11-1 orthe second port 11-2 until a predetermined time elapses after the timer21 is started, the network monitoring unit 29 stores the frame inputanew in the frame storing unit 30, resets the timer 21, and counts timeanew. When a frame is not input to the first port 11-1 or the secondport 11-2 until the predetermined time elapses after the timer 21 isstarted, the network monitoring unit 29 determines that a framedisappears in the network (the communication system), generates tokenframe reissue information, which is a MAC address of a communicationnode to which the transmission right is given next, and passes the tokenframe reissue information to the token-frame processing unit 27. Thetoken frame reissue information is obtained by acquiring, from the tokencirculation order information, a MAC address of a communication nodethat acquires the transmission right after an SA of the frame stored inthe frame storing unit 30.

The frame storing unit 30 stores a frame flowing in the network andinput to the ring management station X. To generate the token framereissue information, it is sufficient if there is an SA of a frameacquired by the ring management station X among frames flowing in thenetwork. Therefore, rather than storing the frame itself, the SA of theframe can be stored. This makes it possible to suppress a capacity forstoring information.

When the token-frame processing unit 27 receives the token frame reissueinformation from the network monitoring unit 29, the token-frameprocessing unit 27 sets, in the MAC address information fortoken-addressed-to-own-station determination of the token frame, thetoken frame reissue information, i.e., a MAC address of a communicationnode to which the transmission right should be given next, generates atoken frame, and transmits the token frame. When the token frame reissueinformation is the MCA address of the own station, the token-frameprocessing unit 27 does not generate a token frame and performsprocessing assuming that the own station obtains the transmission right.Components same as those shown in FIG. 2-1 are denoted by the samereference numerals and signs and explanation of the components isomitted.

The configuration of a slave station is basically the same as thatexplained with reference to FIG. 2-2 in the first embodiment. However,it is assumed that the slave station transmits all output frames inbroadcast or multicast. When the frames are transmitted in multicast, itis assumed that all communication nodes included in the communicationsystem are set as one group in advance. When all the frames aretransmitted in broadcast or multicast, as in the token frame shown inFIG. 3-5, it is assumed that, for example, “MAC address information forframe-addressed-to-own-station determination” is provided in a data areato make it possible to discriminate to which communication node a frameto be broadcasted or multicasted is transmitted.

Reissue processing for a token frame is explained below. FIGS. 16-1 to16-8 are schematic diagrams of an example of a procedure of reissueprocessing for a token frame according to the fourth embodiment. Thiscommunication system includes, as shown in FIG. 16-1, a network of thesame segment including the ring management station X and the slavestations A to C. More specifically, this communication system has aconfiguration in which the slave station A is connected to the firstport X1 of the ring management station X and the slave stations B and Care connected to the second port X2 of the ring management station X ina line shape. It is assumed that circulation order of a token in thiscommunication system is set as indicated by (F) below.

Ring management station X→slave station A→slave station B→slave stationC→ring management station X (F)

First, as shown in FIG. 16-1, in this communication system, after thering management station X that acquires a token frame transmitsnecessary data to the slave stations A to C, the ring management stationX issues a token frame to give the transmission right to the slavestation A. At this point, the network monitoring unit 29 of the ringmanagement station X starts the timer 21.

Subsequently, the slave station A receives the token frame and recognizethat the own station acquires the transmission right. As shown in FIG.16-2, when the slave station A transmits a data frame (in the figure,represented as DataFrame(A→X)) in broadcast, the data reaches the ringmanagement station X. The network monitoring unit 29 of the ringmanagement station X stores a frame input from the first port X1 in theframe storing unit 30 and resets the timer 21. Thereafter, it is assumedthat the token frame circulates and transmission of data normally endsin the same manner in the slave stations B and C. FIG. 16-3 is a diagramof a state after the slave station C normally transmits data. At thispoint, a data frame transmitted by the slave station C is stored in theframe storing unit 30 of the ring management station X. The networkmonitoring unit 29 resets the timer 21.

Thereafter, as shown in FIG. 16-4, the slave station C that transmitsthe data frame transmits a token frame to give the transmission right tothe ring management station X. It is assumed that this token framedisappears in the transmission line 111 between the slave station C andthe slave station B. Therefore, the token frame does not reach the slavestation B and, as a result, does not reach the ring management station Xeither. As shown in FIG. 16-5, a state in which a frame does not flow inthe network lasts until a predetermined time elapses after the timer 21is started.

Subsequently, as shown in FIG. 16-6, the network monitoring unit 29 ofthe ring management station X detects that the timer 21 is time over anddetermines, using the frame stored in the frame storing unit 30 and thetoken circulation order information stored in thetoken-circulation-order-information storing unit 25, to which slavestation data of the slave stations has been able to be normallytransmitted. Specifically, the network monitoring unit 29 of the ringmanagement station X acquires the SA of the frame stored in the framestoring unit 30 and acquires, from the token circulation orderinformation, a MAC address of a communication node to which thetransmission right is given after this SA. The network monitoring unit29 generates the acquired MAC address as token frame reissue informationand passes the MAC address to the token-frame processing unit 27.

In this example, because the data frame transmitted by the slave stationC is stored in the frame storing unit 30, the network monitoring unit 29determines that data transmission normally ends up to the slave stationC. As indicated by the token circulation order (F), the networkmonitoring unit 29 recognizes that the own station (the ring managementstation X) obtains the transmission right after the slave station C,generates token frame reissue information having the MAC address “100”of the own station, and passes the token frame reissue information tothe token-frame processing unit 27.

Because the destination of the token frame is the own station, thetoken-frame processing unit 27 of the ring management station Xrecognizes that the own station obtains the transmission right.Subsequently, as shown in FIG. 16-7, the ring management station Xperforms transmission processing for data. Thereafter, as shown in FIG.16-8, the ring management station X generates a token frame for passingthe transmission right to the slave station A and transmits the tokenframe.

As explained above, when the token frame disappears without reaching thering management station X (when the frame disappears after the dataframe reaches the ring management station X), the network monitoringunit 29 of the ring management station X specifies a communication nodethat normally ends data transmission. A token frame is reissued to acommunication node that obtains the transmission right after thecommunication node.

Another example of the reissue processing for a token frame at the timewhen a token frame disappears during communication performed using thetoken frame is explained. FIGS. 17-1 to 17-5 are schematic diagrams ofanother example of the procedure of the reissue processing for a tokenframe according to the fourth embodiment. In this example, as in theexample explained above, it is assumed that the communication system hasa same configuration as that shown in FIGS. 16-1 to 16-8 and tokencirculation order is the one indicated by (F) above.

First, as shown in FIG. 17-1, after the slave station A that acquiresthe transmission right normally transmits data, the slave station Atransmits a token frame to pass the transmission right to the slavestation B. However, as shown in FIG. 17-1, it is assumed that the tokenframe disappears on the transmission line 112 between the ringmanagement station X and the slave station B. At this point, the networkmonitoring unit 29 of the ring management station X stores the tokenframe issued by the slave station A in the frame storing unit 30 andresets the timer 21.

Thereafter, as shown in FIG. 17-2, when a state in which a frame doesnot flow lasts a predetermined time or more, as shown in FIG. 17-3, thenetwork monitoring unit 29 of the ring management station X detects thatthe timer 21 is time over and determines, using the frame stored in theframe storing unit 30 and the token circulation order information storedin the token-circulation-order-information storing unit 25, to whichslave station data of the slave stations has been able to be normallytransmitted. Specifically, the network monitoring unit 29 of the ringmanagement station X acquires the SA of the frame stored in the framestoring unit 30 and acquires, from the token circulation orderinformation, a MAC address of a communication node to which thetransmission right is given after this SA. The network monitoring unit29 generates the acquired MAC address as token frame reissue informationand passes the MAC address to the token-frame processing unit 27.

In this example, because the token frame transmitted by the slavestation A is stored in the frame storing unit 30, the network monitoringunit 29 determines that the data transmission normally ends up to theslave station A. As indicated by the token circulation order (F), thenetwork monitoring unit 29 recognizes that the slave station B obtainsthe transmission right after the slave station A, generates token framereissue information having the MAC address “2” of the slave station B,and passes the token frame reissue information to the token-frameprocessing unit 27.

Thereafter, the token-frame processing unit 27 of the ring managementstation X generates, based on the token frame reissue information, atoken frame in which the MAC address “2” of the slave station B includedin the token frame reissue information is set in the MAC addressinformation for token-addressed-to-own-station determination. As shownin FIG. 17-4, the token-frame processing unit 27 transmits the tokenframe in broadcast. An SA of this token frame is the MAC address of thering management station X.

Thereafter, as shown in FIG. 17-5, when the slave station B receives thetoken frame from the ring management station X, the slave station Bobtains the transmission right and outputs data of the own station tothe network. The slave station B transmits the token frame to give thetransmission right to the next communication node.

In this way, when the token frame disappears after reaching the ringmanagement station X, the ring management station X specifies acommunication node that normally ends data transmission, i.e., acommunication node corresponding to an SA of a token frame receivedlast. A token frame is reissued to a communication node that obtains thetransmission right after the communication node.

As explained above, the ring management station X monitors disappearanceof a frame in the communication system and stores a frame acquired last.This makes it possible to give the transmission right to a communicationnode at a token circulation destination next to a communication nodethat normally transmits data last.

According to the fourth embodiment, when a frame disappears halfway, atoken frame is not reissued from the ring management station X. Rather,a communication node in which data transmission is normally performedlast before the frame disappearance and a token frame is reissued to acommunication node to which the transmission right is given after thecommunication node. As a result, there is an effect that the ringmanagement station X can receive data from the slave stations A to Cbefore and after the frame disappearance.

Fifth Embodiment

In the fourth embodiment, the reissue processing for a token frameperformed when a frame disappears is explained. For example, when acommunication node in the communication system is left off from thecommunication system because of power-off or the like, it is possible tocope with the left-off using the method explained in the fourthembodiment. Specifically, the ring management station X reissues a tokenframe to the left-off communication node. After the issuance of thetoken frame, when there is no response even when a predetermined timeelapses, a token frame only has to be issued to a communication node towhich the transmission right is given after the left-off communicationnode.

However, in this method, until the left-off communication node is lefton by power-on again, the same reissue processing for a token frame isrepeated every time token frame circulation order comes to the positionof the communication node.

In the Ethernet, communication nodes are often connected in a lineshape. When left-ff occurs in a higher-order communication node close tothe ring management station among the communication nodes connected in aline shape in this way, communication nodes lower in order than thecommunication node cannot perform communication either. In such a case,even if the reissue processing for a token is performed, a frame doesnot flow of the communication system as long as a higher-ordercommunication node is left off. Therefore, useless processing isrepeated until the reissue processing reaches a lowest-ordercommunication node in the line. When a plurality of communication nodesare left off, the reissue processing for a token frame is performed aplurality of times while the token frame circulates once.

In this way, after the ring management station issues a token frame,when no response is received for the predetermined time, if the methodexplained in the fourth embodiment is used to cope with this problem, insome case, the request for real-time properties in the communicationsystem is not satisfied. Therefore, in the fifth embodiment, processingperformed when the communication nodes included in the communicationsystem are left off by power-off is explained.

The configuration of the ring management station in the fifth embodimentis basically the same as that explained in the fourth embodiment.However, the network monitoring unit 29 further has a function ofstarting the timer 21 as well when a token frame is reissued,determining, when transmission of a frame is not performed for apredetermined time from a communication node to which the token frame isreissued, that the communication node is in a power-off state, andinstructing execution of the network presence check processing by thenetwork-presence-check processing unit 22.

FIG. 18 is a schematic block diagram of an example of a functionalconfiguration of a slave station according to the fifth embodiment. Theslave station further includes, in the configuration shown in FIG. 2-2in the first embodiment, a timer 65 and a reset processing unit 66 in acommunication processing unit 60A.

The timer 65 is started by the reset processing unit 66 and has afunction of measuring a predetermined time. In the fifth embodiment, thetimer 65 counts time until the predetermined time elapses after datafrom the ring management station X is received by thedata-frame-communication processing unit 28. The data from the ringmanagement station X can be, for example, transfer of the transmissionright or update data transmitted from the ring management station X tothe slave station. It is assumed that a period of the update datatransmitted from the ring management station X to the slave station ismeasured.

The reset processing unit 66 starts the timer 65 when, for example, datato be updated from the ring management station X to the slave station isreceived, determines, when the update data from the ring managementstation X is not received for the predetermined time, that the ringmanagement station X is in an absent state, and performs resetprocessing for erasing the token circulation destination informationstored in the token-circulation-destination-information storing unit 62.Time of the timer 65 set by the reset processing unit 66 is set longerthan an average time in which a token frame circulates in thecommunication system once and set to about the double of time of thetimer 21 started by the network monitoring unit 29 of the ringmanagement station X.

The reconfiguration processing for a logical ring is explained below.FIGS. 19-1 to 19-9 are schematic diagrams of an example of procedures ofthe determination processing for leaving-off the slave station and thereconfiguration processing for a logical ring according to the fifthembodiment. In this example, as in the example explained above, thecommunication system has a same configuration as that shown in FIGS.16-1 to 16-8. Token circulation order is that indicated by (F) above.

First, as shown in FIG. 19-1, in a state in which communication isperformed by circulating a token frame, the ring management station Xreceives the token frame, acquires a transmission right, and transmitsdata to the slave stations A to C. When the reset processing units 66 ofthe slave stations A to C receive the data from the ring managementstation X, the reset processing units 66 start the timers 65.Thereafter, as shown in FIG. 19-2, it is assumed that the power supplyfor the slave station C is turned off.

Subsequently, as shown in FIG. 19-3, the token frame circulates to theslave station B. After transmitting data of the own station, the slavestation B transmits the token frame to pass the transmission right tothe slave station C. At this point, the network monitoring unit 29 ofthe ring management station X stores this token frame in the framestoring unit 30 and resets the timer 21. Because the power supply forthe slave station C is off, the slave station C cannot receive the tokenframe. As a result, as shown in FIG. 19-4, a state in which a frame doesnot flow in the communication system lasts.

Thereafter, as shown in FIG. 19-5, when the state in which a frame doesnot flow lasts for the predetermined time or more, the networkmonitoring unit 29 of the ring management station X detects that thetimer 21 is in a state of time over and determines, based on informationof the frame stored in the frame storing unit 30, to which slave stationdata has been able to be normally transmitted. Because the token frametransmitted by the slave station B is stored, the network monitoringunit 29 determines that data of the slave stations has been able to benormally transmitted up to data of the slave station B and recognizes,using the token circulation order information of thetoken-circulation-order-information storing unit 25, that a circulationdestination of the token frame after the slave station B is the slavestation C. As shown in FIG. 19-6, the ring management station Xgenerates a token frame for passing the transmission right to the slavestation C and transmits the token frame. At this point, the networkmonitoring unit 29 of the ring management station X starts the timer 21again.

As explained above, as shown in FIG. 19-7, because the power supply forthe slave station C is in the off state, the slave station C cannotreceive the token frame. The state in which a frame does not flow in thecommunication system lasts after the ring management station X transmitsthe token frame for giving the transmission right to the slave stationC.

Thereafter, as shown in FIG. 19-8, when the state in which a flow doesnot flow lasts for the predetermined time or more, the networkmonitoring unit 29 of the ring management station X detects that thetimer 21 is in a state of time over. The network monitoring unit 29detects that a frame is not transmitted from the slave station C at anissue destination of the token frame, determines that the slave stationC is left off from the communication system, and instructs thenetwork-presence-check processing unit 22 to execute the networkpresence check processing.

On the other hand, as shown in FIG. 19-8, when a state in which updatedata from the ring management station X is not received lasts for thepredetermined time, the reset processing units 66 of the other slavestations A and B detect that the timers 65 are in a state of time overand perform reset processing for erasing the token circulationdestination information stored in thetoken-circulation-destination-information storing unit 62.

Thereafter, as shown in FIG. 19-9, the network-presence-check processingunit 22 of the ring management station X transmits a network presencecheck frame. As shown in FIG. 19-10, because the token circulationdestination information of the token-circulation-destination-informationstoring unit 62 is erased, the control-frame response units 61 of theslave stations A and B perform response processing to the networkpresence check frame transmitted from the ring management station X. Asexplained in the first embodiment, the control-frame response units 61perform the processing for configuring a logical ring (thereconfiguration processing for a logical ring).

In the above explanation, the timers 65 are provided in the slavestations A to C. When the update data is not received from the ringmanagement station X for the predetermined time or more, the resetprocessing units 66 erase the token circulation destination informationof the token-circulation-destination-information storing unit 62.However, the token circulation destination information of the slavestations A to C can be erased by other methods. For example, it is alsopossible that, when the network monitoring unit 29 of the ringmanagement station X determines that leaving-off of a slave stationoccurs, the network monitoring unit 29 generates a reset frame forforcibly causing all the slave stations to forget the token circulationdestination information and the data-frame-communication processing unit28 transmits the reset frame to the communication nodes in thecommunication system in broadcast.

According to the fifth embodiment, when a frame stops flowing in thecommunication system during transmission and reception processing fordata performed using a token frame, the ring management station Xtransits the token frame to the communication node next to acommunication node to which the data is normally transmitted.Thereafter, when a frame does not flow in the communication system evenwhen the predetermined time elapses, the ring management station Xdetermines that a communication node at a reissue destination of thetoken frame is left off because of a reason such as power-off andreconfigures a logical ring. As a result, for example, there is aneffect that communication can be resumed earlier compared with the timewhen a token frame is reissued to the communication node next to thecommunication node determined as being left off.

In particular, in the Ethernet, in general, communication nodes areconnected in a star shape. However, as an industrial network, saving ofwires is requested and communication nodes are often connected in a lineshape. When a communication node close to a ring management station sideof the communication nodes connected in a line shape in this way is leftoff, all the communication nodes lower in order than the communicationnode cannot perform communication. In such a case, as explained above,when there is no response even if a token frame is reissued to theleft-off communication node using the method in the fourth embodiment,there is no response even if a token frame is reissued to acommunication node having the transmission right after the left-offcommunication node. when power supplies for a plurality of communicationnodes are turned off, processing for reissuing a token frame to acommunication node next to a communication node, a power supply forwhich is turned off, is repeatedly performed a plurality of times andreal-time properties of communication are spoiled. In such a case, asexplained in the fifth embodiment, the reconfiguration processing for alogical ring is performed. This makes it possible to minimize a delayand perform communication having real-time properties in a new networkconfiguration.

The embodiments explained above can be applied to a communication systemsuch as an FA system in which real-time properties of communication arerequested.

A data communication method in the ring management station and the slavestation explained above can be realized by executing, with a computersuch as a programmable controller or a personal computer including a CPU(central processing unit), programs in which respective processingprocedures are written. In this case, the CPU (control means) of thecomputer executes the processing steps of the data communication methodexplained above according to the programs. These programs are recordedin a computer-readable recording medium such as a hard disk, a floppydisk, a CD (Compact Disk)-ROM (Read Only Memory), an MO (Magneto-Opticaldisk), or a DVD (Digital Versatile Disk or Digital Video Disk) andexecuted by being read out from the recording medium by the computer.These programs can also be distributed via a network (a communicationline) such as the Internet.

Further, the ring management station can be a communication managementcircuit obtained by realizing the processing units described in theembodiments with a circuit that executes processing according to theprocessing procedures. Similarly, the slave station can also be acommunication circuit obtained by realizing the processing unitsdescribed in the embodiments with a circuit that executes processingaccording to the processing procedures.

Furthermore, the ring management station can be an LSI (Large-ScaleIntegration) obtained by manufacturing the processing units described inthe embodiments to execute processing according to the processingprocedures. Similarly, the slave station can also be an LSI obtained bymanufacturing the processing units described in the embodiments toexecute processing according to the processing procedures.

INDUSTRIAL APPLICABILITY

As explained above, the data communication system according to thepresent invention is useful for a network system connected by theEthernet in which real-time properties of data communication arerequested.

1. A communication management apparatus that manages transmission ofdata in a network in which a plurality of communication nodes areconnected in a star shape or a line shape by an Ethernet, thecommunication management apparatus comprising: a network-presence-checkprocessing unit that performs a network presence check processing ofacquiring information concerning connection between the communicationnodes from the communication nodes; a token-circulation-orderdetermining unit that determines token circulation order indicating anorder of circulating a token frame for acquiring a transmission right ofdata between the communication nodes using information acquired by thenetwork presence check processing; a setup processing unit thatnotifies, based on the token circulation order, each of thecommunication nodes in the network of token circulation destinationinformation indicating a communication node that acquires thetransmission right next; a data-frame-communication processing unit thatreceives a data frame from other communication nodes and, when thetransmission right is acquired, transmits the data frame; and atoken-frame processing unit that determines whether the transmissionright is obtained by analyzing the received token frame and, when thetransmission right is obtained, after transmission of a data frame bythe data-frame-communication processing unit, transmits a token frame inwhich, based on the token circulation order, information on acommunication node that acquires the transmission right next is set. 2.The communication management apparatus according to claim 1, wherein thesetup processing unit notifies each of the communication nodes in thenetwork of the token circulation destination information and, afterconfirming that a response is received from all the communication nodesthat are notified of the token circulation destination information,instructs the token-frame processing unit to generate the token frame.3. The communication management apparatus according to claim 1, wherein,when the own apparatus receives the transmission right a predeterminednumber of times, the network-presence-check processing unit further hasa function of performing the network presence check processing.
 4. Acommunication node arranged in a network in which a plurality ofcommunication nodes are connected in a star shape or a line shape by anEthernet cable, the communication node performing transmission of datawhen the communication node acquires a token frame fed according totoken circulation order determined by a communication managementapparatus, which is one of the communication nodes, the communicationnode comprising: a token-circulation-destination-information storingunit that stores information concerning a token circulation destinationthat is a communication node to which a transmission right is givenafter the own apparatus; a token-frame processing unit that comparestransmission right acquiring apparatus information indicating acommunication node that acquires the transmission right the next, whichis included in the token frame received from the other communicationnodes, and an address of the own apparatus, determines whether thetransmission right is obtained, and, when the transmission right isobtained and transmission processing for a frame by adata-frame-communication processing unit ends, transmits a token framein which the token circulation destination information stored in thetoken-circulation-destination-information storing unit is set as thetransmission right acquiring apparatus information; and adata-frame-communication processing unit that transmits a data framewhen the transmission right is acquired and receives a data frame fromthe other communication nodes.
 5. A data communication method for aplurality of communication nodes in a communication system, thecommunication system including a communication management apparatus thatis one of the communication nodes and manages transmission of data in anetwork in which the communication nodes are connected in a star shapeor a line shape by an Ethernet cable and slave stations that are othercommunication nodes in the network, the data communication methodcomprising: transmitting a token frame in the network in broadcast, thetoken frame storing transmission right acquiring apparatus informationthat is a communication node that acquires a transmission right afterthe own communication node according to token circulation orderdetermined by the communication management apparatus by expanding, in atree shape, a connection relation among the slave stations connected tothe communication management apparatus and selecting in order thecommunication nodes connected in the tree shape in such a manner as toturn around a tree from the communication management apparatus; andreceiving the token frame, determining whether the transmission rightacquiring apparatus of the transmission right acquiring apparatusinformation included in the token frame is the own apparatus, when thetransmission right acquiring apparatus of the transmission rightacquiring apparatus information is the own communication node,determining that the own communication node acquires the transmissionright and transmitting a data frame, and, when the transmission rightacquiring apparatus of the transmission right acquiring apparatusinformation is not the own apparatus, repeating the token frame, whereinthe transmitting the token frame and the receiving the token frame arerepeated to circulate, in order of the token circulation order, thetoken frame to all the communication nodes connected to the network. 6.The data communication method according to claim 5, wherein, in thetransmitting the token frame, in the case of the communicationmanagement apparatus, the transmission right acquiring apparatusinformation is acquired from the token circulation order and, in thecase of each of the slave stations, the transmission right acquiringapparatus information is acquired from information concerning a tokencirculation destination that is a communication node to which thetransmission right is give after the own slave station notified inadvance by the communication management apparatus.
 7. The datacommunication method according to claim 5, further comprising, beforethe transmitting the token frame: the communication management apparatustransmitting, when a power supply is turned on, a network presence checkframe including an address of the own apparatus in broadcast to theslave station present in the network; the slave station generating, whenthe network presence check frame is received, a network presenceacknowledge frame including a transmission source address of the networkpresence check frame and addressed to an address of the communicationmanagement apparatus included in the network presence check frame,transmitting the network presence acknowledge frame, rewriting thetransmission source address of the received network presence check frameto an address of the own station, and transferring the network presencecheck frame; the communication management apparatus generating, when thenetwork presence acknowledge frame is received, network presenceinformation including a transmission source address and the transmissionsource address of the network presence check frame received by the slavestation from the network presence acknowledge frame; the communicationmanagement apparatus determining, when a predetermined time elapsesafter the network presence check frame is transmitted, the tokencirculation order by using the transmission source address of thenetwork presence check frame in the network presence information and thetransmission source address of the network presence acknowledge frame;the communication management apparatus transmitting in broadcast, afterthe determination of the token circulation order, a setup frame fornotifying each of the slave stations in the network of informationconcerning a token circulation destination that is a communication nodeto which the transmission right is given after the slave station; theslave station storing the token circulation destination information whenthe setup frame is received and transmitting a setup response frame tothe communication management apparatus; and the communication managementapparatus generating the token frame according to the token circulationorder when the setup response frame is received from all thecommunication apparatuses in the network.
 8. The data communicationmethod according to claim 7, further comprising: the communicationmanagement apparatus storing information including a transmission sourceaddress of a frame input to a port of the own apparatus; and thecommunication management apparatus acquiring, when a frame is not inputto the port of the own apparatus for a predetermined time or more, fromthe network circulation order, a communication node that obtains thetransmission right after the stored transmission source address of theframe and reissuing a token frame to the acquired communication node. 9.The data communication method according to claim 8, wherein, in thereissuing a token frame, when a communication node that obtains thetransmission right next is the own apparatus, the communicationmanagement apparatus transmits, without reissuing a token frame, a dataframe assuming that the transmission right is acquired.
 10. The datacommunication method according to claim 8, wherein, after the reissuinga token frame, when a frame is not received for a predetermined time ormore from the communication node to which the token frame is reissued,the communication management apparatus shifts the processing to thetransmitting a network presence check frame.